In one sense there is no limit to a Christological hermeneutic as it is arguable ‘the’ New Testament hermeneutic of the scriptures. In some ways it is more of a Biblical attitude or higher framework directing other hermeneutics in how biblical truth theologically fits into an understanding of self, God and our relationship to him. It is not a precise guide for interpreting an individual historical literal meaning of a given verse of scripture which could be done without faith to a degree. It is more of a belief that if one correctly and honestly uses a historical grammatical exegesis, one will arrive at a Christological conclusion.
Here we see a Christological attitude and framework in the New Testament itself:
For I resolved to know nothing while I was with you except Jesus Christ and him crucified.( The New International Version. 2011 (1 Co 2:2). )
May I never boast except in the cross of our Lord Jesus Christ, through which the world has been crucified to me, and I to the world. (The New International Version. 2011 (Ga 6:14))
Jews demand signs and Greeks look for wisdom, 23 but we preach Christ crucified: a stumbling block to Jews and foolishness to Gentiles, 24 but to those whom God has called, both Jews and Greeks, Christ the power of God and the wisdom of God.( The New International Version. 2011 (1 Co 1:22–24))
In many ways ‘this higher framework’ under the tradition of St. Augustine, was in many ways lost among medieval scholasticism. Then theology was basically a mixture of Aristotelian philosophy with Church Dogma. The result is that the main points of salvation topics were blurred taking the emphasis away from sin and salvation by faith in Christ. The 'Christological hermeneutic' resurfaced in Luther’s own lectures and can be seen very early in his theological career even before the Protestant reformation. This hermeneutic is still central to most protestant creeds of faith today. The basic idea is that as we are truly and seriously born in sin without human remedy. We are also without understanding and without ability to understand. As God seeks remedy for our depravity and ignorance, through the death of Christ, we can obtain knowledge, righteousness and life. This by simple faith in that work done on the cross. A 'Christological hermeneutic' is actually a Cross-ological hermeneutic for it is not just about philosophical topics about the incarnation but rather how having faith in the incarnation and penal crucifixion of Christ we can obtain personal crucifixion and resurrection of our own selves. Therefore, understanding scripture is part of understanding God and self, including an existential experience of its power, which can only be done by faith in Christ. This makes the scripture themselves Christological.
One can see for example this attitude in Luther’s first formal exposition of scripture in his lectures on the Pslams. This was the time when all his ideas were fermenting but not yet resolved around the doctrine of justification which wold emerge as his only passion.
THE FIRST PSALMS LECTURE (1513–1515)
THE NEW HERMENEUTIC
Compared with the marginal notes of 1509/1510, the first Psalms lecture reflects a totally different theological climate. The reason for this is first of all that we are dealing here with Luther’s first exegetical lecture, no longer with mere criticisms or comments on the texts of others. In addition, the first Psalms lecture makes clear that while his interpretation to a great extent leaned on earlier expositions, of which Augustine’s Ennarationes in Psalmos have the lion’s share, in a hermeneutical respect Luther was going his own way…The new hermeneutic appears in its essential features as early as in Luther’s preface. If earlier commentators had dealt with questions of introduction and hermeneutics in their forewords, Luther’s superscription reads: “Foreword of Jesus Christ.” A few quotations from Scripture intend to show that Christ is the sole key to understanding the Psalter. (Lohse, B. (1999). Martin Luther’s Theology: Its Historical and Systematic Development (R. A. Harrisville, Trans.) (52–53). Minneapolis, MN: Fortress Press.)
This hermeneutic gets at the meat of virtually all Protestant theology. It guides everything that is ‘theologically relevant’ to an understanding of God and our own self with a view of salvation or damnation. It is personally serious not academic. Many other branches of biblical study and theology virtually ignore this central Christological theme, which the Protestant reformation revolted from in the form of a renewed Christological Hermeneutic.
But to get back to the precise question: ‘But when it comes to the practicalities of understanding the Bible, how does that work?’ It has to do with the goal of understanding the Bible and the assumed purpose for the Bible. When we assume that we are lost and needing Salvation and that the Bible is provided as a guide for us to find out how to be saved, then Christ is put at the center of this view and all biblical interpretation flows into it. If we do not make these assumptions (i.e. belief in strict original sin, atonement for sin and imputation of an alien righteousness, through Christ’s death and resurrection) then the hermeneutic is entirely misleading and absolutely limited or false. On the other hand, if we do have these Protestant assumptions or adhere to the hermeneutic about Christ’s role in our salvation, then it has an impact on the accent and direction of virtually every Biblical narrative and intended final application of every biblical thought and verse.
I know allegories are not often that helpful but I think it might be here. Put it this way: If we dumped a big box of iron shavings on a wooden table they would all fall in random order. Each shaving could be analyzed and measured objectively. We could all agree that this shaving is 2 cm long, black, of a certain weight, etc. However, if we then installed a strong magnet below the table each strand and shaving would all change direction to the magnetic pole. If we consider only those shavings 'directly' over the magnet to 'directly' mention Christ, then all the other shavings do not seem to be practically affected by the hermeneutic. However, if we pay careful attention to the orientation or angular relationship to the other shavings and those liberally over the location of the magnet, we find almost everything is changed! Some shavings may have been accidentally aligned to the magnetic poles before the magnetic force was applied, but even these probably undergo some modification when looked at closer. Under this analogy, the word of God, is the logos, which is Christ. It's all related by something outside of history, grammar and even human understanding. Christ is where all truth, thought and scripture point because God communicates his mind to us in love for our salvation. Everything else is as Solomon would say, 'meaningless, meaningless'.