It's from the Japanese saying, 読書百遍意自ずから通ず(Repeated reading makes the meaning clear). In this case, the ず ending doesn't make the verb negative. What is the function and nuance of this usage? Is this a verb form from the old days reserved for use in Japanese proverbs and perhaps some other specific and narrowly defined instances? Another example where this is used in the same way is 窮すれば通ず [きゅうすればつうず] (Necessity is the mother of invention). Is there another example with another verb that uses the ず ending in the same way?
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I'm guessing it derives from
通ず comes from the verb 通ずる (to lead to). In this case, ず is used as another way to conjugate the 〜する/〜ずる verb.
In answer to your question:
Here are just a few examples based off of this post by urutorasenpaiさん where this ず can commonly be seen:
Seeing that 〜ず, in this case, seems to happen most often in ことわざ or classical Japanese would seem to indicate that this is possibly more of a poetic, stylistic conjugation and use of the verb. It helps with fitting verbs into a smaller amount of syllables, as well.