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? 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 i