Rashi: It is a Halachah sealed in a Chacham's heart.

R. Yonah: You will find correct counsel and clear Da'as in a person only with extended thought and contemplation of the kidneys ¹ , just like a person can drink from deep water only via a rope and a bucket. The verse comes to distance from the way of those who trust in their thinking and the first matter to cross their Ru'ach, without deliberation and investigation in the heart. One sees correctly only after deep thought and investigating Da'as. Before it discussed an Atzel regarding work; here it distances laziness also regarding investigation.

Malbim: Etzah is to choose what is good and beneficial from all possibilities. Many thoughts deluge on the heart like gushing water - "Rabos Machashavos b'Lev Ish" (19:21). Choosing the counsel that will lead to the goal is hidden in the depth of the heart. There is Etzah from Chachmah and Etzah of Hashem, who 'bequeathed' the laws of Chachmah. One receives this Etzah from others. People received Chachmah - Shome'a l'Etzah Chacham" (12:15). "Lo Sovad Torah mi'Kohen v'Etzah me'Chacham" (Yirmeyah 18:18).

Rashi: A clever Talmid draws the Halachah from the Chacham's innards.

Malbim: Etzah found in man's heart itself, that he understands from his Da'as to choose this good, is very deep. One draws it from the depth of the Nefesh only via the power of Binah, understanding a matter amidst another matter. Via this he judges all hidden parts to see which is best. Binah's investigation via comparisons is like drawing water from a deep place. He ties rope to rope until he lowers the flask to the spring. So he ties comparison to comparison until he takes out the Emes with the power of Binah.