If you look at Proverbs 6:16 and Proverbs 30:18, they are written in a certain format:
"There are six things the LORD hates--no, seven things he detests"
"There are three things that are too amazing for me, four that I do not understand"
Why was this written this way? Is it some technique of Hebrew poems?
With above sentences the writer (not sure if this is Agur or the son of Jakeh) is talking about the 7th and 4th item. For example: 3 things amazes him and there is even a 4th thing that amazes him even more.
The book of Proverbs is completely in poetic form. It contains different variations and combinations of basic forms of parallelism, a distinguishing feature of Hebrew poetry.
Traditional English poetry is characterized by rhythm and the rhyming of final words of lines. Translation of rhymed poetry into another language is almost impossible.
However, Hebrew poetry was not built on rhyming words but on parallel ideas. The technical term is parallelism (see Parallelism in Hebrew Writing). Hebrew poetry is constructed of two lines with some pre-designed relationship between each line.
So on your question:
Is this a figure of speech?
Mostly yes because figure of speech is one of the basics of parallelism.
OP: Is this a figure of speech?
No, the "graded numerical sayings" are not "figures of speech".
It is more accurate to describe this "n + 1" pattern in biblical poetry in terms of "rhetoric" (or "stylistics") rather than a "figure of speech" which normally has to do with non-literal language (e.g., metaphor, simile).
Although "graded numerical sayings", as they are sometimes called, are found not only in wisdom literature (like the examples given by OP, and Proverbs 30 has a nice collection of them), but also the prophets (especially Amos 1-2), they are also found in the literature of Ugarit and in the cuneiform texts from Mesopotamia.
As another answer helpfully points out, it is often thought that the last line (the "climax" of the number of things) is the "punch-line", although there are cases like the Amos examples where the "climactic" aspect is subdued or absent. The origins of this saying-type are not certain, but the common usage suggests a kind of ancient social science, observing and classifying nature and behaviour.
An older study of this type of saying by W.M.W. Roth, Numerical Sayings in the Old Testament: A Form-Critical Study (Brill, 1965) is available with a preview on Google Books, and most newer handbooks on biblical wisdom will offer some comment on this saying-type as well.
Why was this written this way? Is it some technique of Hebrew poems?
A figure of speech is language that expresses one thing in terms of another by analogy, extension, or other association. There are four kinds of figures of speech: image, symbol, simile, and metaphor. Image represents sensory experience; symbol stands for something not mentioned in the verses, metaphor compares one thing to another, and simile compares one thing to another with the use of like or as.
"There are six things the LORD hates--no, seven things he detests"
"There are three things that are too amazing for me, four that I do not understand"
Are there figures of speech in these verses?
Is there sensory experience in these lines? something that stands for something else? a comparison of one thing to another with or without like or as?
Conclusion: There are no figures of speech in these verses.
