Sensus plenior is used by different people in different ways. The most common meaning used in debates among Catholics and later among Evanglicals is the one offered by Raymond Brown:
The debates were done by men who admittedly did not know how to do sensus plenior. see  If you had debates held on the genre of riddle, by men who had never seen nor solved a riddle, the conclusions would be the same as for sensus plenior.
Sensus plenior is not interpreting scriptures by modern methods. Those who claim to be able to unpack sensus plenior claim that the apostles use the method within the gospels to teach how to do it.
Sensus Plenior, though not a threat to the doctrines of normative protestantism, is a threat to professional practitioners of the same. If sensus plenior exists in the form of a spiritual layer on top of the literal, the the practitioner is only familiar with half of the source material for his profession. If it exists in four layers, where the spiritual layer contains three voices, then the professional is only familiar with 1/4 of the source material for his profession. The field of hermeneutics is largely influenced by the same group of people who are placed most at risk by the introduction of sensus plenior to the discussion.
Though sensus plenior interpretations will appear to be allegory it must be emphasized that a strict set of rules are followed which are so demanding that no human interpreter is permitted to do free-for all allegory. The most important rule which locks sensus plenior to the text is that when a word takes on an allegorical meaning, it must be the same everywhere the word is used. Every donkey is a prophet, every garment is works.
It is self-correcting by such rules. For instance, popular typology defines leaven as sin, yet sin does not fit when we are told that the kingdom of heaven is like leaven. Leaven is better defined as 'teaching' where it can be used everywhere it exists.
The common statement that "doctrine drives exegesis" is a patent admission to the subjective nature of modern exegesis. Sensus plenior eliminates that since it holds the practitioner accountable to formal rules.
Compare this sensus plenior exegesis with that of any commentary on Gen 38. Though no attempt is made here to document all the scriptural evidence for each point, the parallels between the birth of Christ and the Tamar narrative are unmistakable. Each metaphoric meaning is subject to the formal rules.
Birth of Christ in Gen 38
Tamar and Mary were in a similar predicament. Tamar has been discovered to be with child and was going to be killed. (Gen 38:24) Mary was discovered with child and was going to be put away. (Matt 1:18-19) When the true father of each child was discovered, they both were honored. (Gen 38:26, Luke 1:42)) Such a coincidence.
Each had wanted an assurance of a promise. Tamar had received three things from Judah (Gen 38:18) and Mary had received three saying from the Angel that announced her pregnancy. (Luke 1:35) Tamar received a staff, Mary was told that the power of the Highest would overshadow her. Tamar received a signet ring, Mary was told he would be called the Son of God. These are easy parallels to understand, but they are easy to dismiss as accidents of history.
Tamar met Judah before Timnath which means the appointment. (Gen 38:14). I will suggest that Mary met God before the appointed time of Christ. Since “It is appointed unto man once to die…” (Heb 9:27) Christ’s appointment was with the cross, and Mary was available to God just before that time.
Tamar was offered a goat by Judah. (Gen 38:17) And Mary was offered THE scapegoat, since the angel told her “He will save his people from their sins.” (Mt 1:21) In the ritual of the scapegoat, two goats are presented. They represent Jesus before and after the cross. One is killed for a burnt offering. In the burnt offering, the priest do not get to eat any part of it since it is completely consumed by fire. It represents the Son’s total devotion to the Father which we cannot participate in, but only stand and watch in awe. The smell of the burnt offering is a sweet savor unto the Lord si nce there is not hint of sin involved in it. You can see the first instance of a burnt offering is when Abraham was willing to sacrifice his son Isaac AND Isaac was willing to be obedient to his father. (Gen 22:2 )
The second goat is released into the wilderness to carry away the sins of the people. It represents Christ’s resurrection, which demonstrates that his sacrifice for us was accepted.
Since Jesus is the fulfillment of the scapegoat images, Mary had also been promised a goat.
The virgin birth
Mary was told that she would conceive miraculously by the Holy Ghost. (Luke 1:35) And when Judah told Tamar he would send a goat, the word he used also means 'sow'. Since Judah represents God in the imagery, and Tamar represents Mary, we have a parallel passage that God would sow the goat.
The need for assurance
Tamar wanted an assurance that she would receive the goat, (Gen 38:17) and Mary also asked for an assuring word of explanation. (Luke 1:34)
Tamar was given the staff or rod which represents the power of God in discipline. (Pr 22:15, Pr 23:13, Pr 29:15) and Mary was told the power of the Highest would overshadow her. (Luke 1:35) She was not being disciplined, but God’s discipline upon mankind as borne by His Son and her son, overshadowed her life.
Tamar was given the signet ring. (Gen 38:18) And Mary was told that he would be called the Son of God. (Luke 1:35)
Tamar was given bracelets. (Gen 38:18) In a very literal sense, bracelets were given to signify a marriage, much like we now give rings. But the real clue to the hidden meaning comes from the law of cleanliness:
Numbers 19:15 And every open vessel, which hath no covering [bracelets] bound upon it, is unclean.
Paul was called a chosen vessel (Acts 9:15) and in other images we find that we are vessels as well. Since Tamar was a vessel that had bracelets, she was not unclean. Likewise Mary was told that she was not unclean because although she was with child, it was by the Holy Ghost. (Luke 1:35)
Tamar bore twins (Gen 38:27) and Mary bore the dual-natured God-Man. (John 8:58)
The names of the twins mean breaking forth and rising sun while Jesus was called Dayspring.
Usurping second son
Have you ever wondered why so many second sons in the Bible got the inheritance rather than the first sons as tradition demands? It is a prophecy that Christ will be the second man, the second Adam. (1Co 15:47, 1Co 15:22)
The image of Tamar’s twin sons would be incomplete if the second son did not get the inheritance, and sure enough, the wrestle in the womb, and the second son emerges first and receives pre-eminence. (Gen 38:28-29)
And so it is with Jesus. In the flesh he died desolate. As God’s only begotten son, he did not succeed in being fruitful and multiplying. Only in his resurrection, as the mystery second son, did he receive the full blessing and inheritance given to Abraham. (Gen 12:2)
Three chances to get it right
God wants living children, not children dead in trespass and sin.
Judah had three sons: Er, Onan and Shelah. Er represents Adam. Adam was alive until he ate the forbidden fruit and he died. (Gen 2:17) (Be careful… he died the day he ate the fruit or you call God a liar). Since he had children in his image and likeness, (Gen 5:3) all his children were also dead. (1Co 15:22, Ro 5:14)
But in riddle Er means 'awake'. Awake is the oppsosite of asleep... and if you are asleep, you are dead and in the dust... so Er was called "Alive from the dust" as was Adam.
Adam blew it. And God killed Er because of sin. (Gen 38:7)
Onan was mistakenly called 'vigorous'. We say it was a mistake because he was named by his mother and all the females of the Bible represent those who do not see clearly (for Eve was deceived). It was a mistake to call him vigorous because he too was dead as was Israel, God's son who was supposed to fulfill the Leverate law on behalf of Adam.
God chose Israel through Abraham to bear living children (Jer 7:22-23) But Israel pursued the flesh and refused to do their duty to God in bearing living children, instead wasting their seed in the earth… (Deut 1:26, Deut 1:43, Deut 8:20, Jud 2:17, 2 Ki 17:14, 2 Chron 24:19, 2 Chron 33:10, Ne 9:29, Ps 81:11, Isa 28:12, Isa 30:15, Isa 42:24, Jer 13:11, Jer 29:1, Eze 20:8, Mt 23:37, Lu 13:34)
…just as Onan did. (Gen 38:9)
Israel blew it.
Not given a chance
The genealogy in Matthew tells us that Joseph, the husband of Mary, is the rightful heir to the throne of David. But he is not given the chance to bear living children. When it is his turn, God himself steps in to bear the first fruits of living children. (1Co 15:20) Likewise, Shelah is not given the chance to bear a son when the time was right. (Gen 38:11, 14)
Smite the earth with a curse
John the Baptist fulfilled the prophecy of the returning Elijah. Elijah had made the rain stop until he spoke. (1Ki 17:1) Since water represents the word of God, there was no word of God for four hundred years until John spoke. His task in fulfilling the prophecy was to turn the hearts of the fathers to the children and of the children to the fathers. (Lu 1:17)
The consequence of failure was that the earth would be smited with a curse. (Mal 4:6)
In the narrative of Tamar, Judah was going to Timnath. But why?
Gen 38:12 … and went up unto his sheepshearers to Timnath, he and his friend Hirah the Adullamite. 13 And it was told Tamar, saying, Behold thy father in law goeth up to Timnath to shear his sheep.
The word for shear is cut off . Hirah means noble family and Adulamite means justice of the people. God was going to the appointed time to cut off his sheep.
God threw Adam out of the garden, killed all but eight in a flood, killed a whole generation of Israel in the desert and now was going up to the appointed time to cut off the sheep. You may be looking forward to some future judgment, but the time of the cross was the time of the final judgment where God would have cut off all the sheep had it not been for Christ, the Lamb of God. When he bore all our sin on the cross what more could be judged?
One thing you never want is justice. Always plead for mercy. God was ready to give men justice, and He did in Christ who reconciled grace and
Have you ever wondered why there were so many desolate, barren women mentioned in the Bible? It’s prophecy! The world was full of dead children, dead in trespass and sin. It was long waiting for the fulfillment of the prophecy given to Eve. (Gen 3:15) The time was right as Tamar removed her widows garments. (Gen 38:14) And the time was right as Mary was selected to be the mother of Jesus. (Luke 1:69-70)
The clift of the rock
When Moses was in the physical presence of God he was hidden in the clift of the rock. (Ex 33:22) Tamar hid herself with a veil. The word is pronounced tsaw-eef. Is it an accident that there is a word pronounced saw-eef that means clift?
The rock is Christ. In sensus plenior, all the rocks are Christ and they are all one rock. In this reasoning, the rock that was split in the desert, (Ex 17:6) is the same rock that Moses was hidden in. It was split by being smitten by the rod (of discipline of God). The real horror of the cross was not the physical agony of death. God himself was split as the Father removed himself from His Son on the cross. When Moses was hidden in the clift of the rock, it represented trusting in the cross to preserve you in the face of God.
As Tamar covered herself with the veil, it tells us that Mary was covered with the grace of the cross in the presence of God.
The death of Christ
Gen 38:19 And she arose, and went away, and laid by her vail from her, and put on the garments of her widowhood.
Mary would be a barren mother again before she would see the first fruits. Jesus had to die before he would be fruitful. (Joh 12:24)
Did not possess the goat.
Tamar did not take possession of the promised goat. (Gen 38:20) Just as Mary knew that Jesus had to be about his Father’s business. (Luke 2:49)
God’s only begotten son
Gen 38:26 And he knew her again no more.
He bore our sin
The two sons together represent Christ. The first son represents Jesus in the flesh who bore our sin. The narrative identifies the first son as the one who wore the scarlet thread. Many may debate it, but with all the evidence presented thus far, the scarlet thread represents our sin. (Isa 1:18) The resurrected Christ, the Son who was fruitful and multiplied, does not have the scarlet thread because Christ died once for our sin. (Heb 9:27, Heb 6:6)
The sensus plenior always speaks of Christ and is in agreement with the literal meaning of scripture as a whole. As the saying goes, the OT is Christ concealed, the New Christ revealed. Once he has been revealed, we can see him hidden in the Old.
It is pretty difficult to support a claim that God did not intend for us to see Christ in these scriptures. Sensus plenior, as practiced above, gives an added dimension to the knowledge we have in the literal scriptures.
: "It is interesting to note that the proponents of the SPlen tend to confine their discussion of this sense to the theoretical plane, seldom appealing to it in their works of exegesis” (The Jerome Biblical Commentary, 1968, 618).
: "Thus we can summarize that on the question of surplus biblical meaning Raymon Brown [who popularized the term] consistently supported the intention and insight (even as he abandoned the mechanics) of the sensus plenior. (Life in abundance: studies of John's Gospel in tribute to Raymond E. Brown, S.S., John R. Donahue, p213)
 "Another way of expressing the differences in qualifications is to point out
that NT writers were directly inspired by God. Current interpreters of Scripture are
not. That allow ed the NT authors certain prerogatives that readers of Scripture do
not presently enjoy. Through direct revelation from God biblical writers could
assign applications based on additional meanings assigned to OT passages." (The New Testament Use of the Old Testament, Robert L. Thomas, TMSJ 13/1 (Spring 2002) 79-98, p.18