How do they hide Da'as?
Rashi: They guard it in their hearts, lest they forget it.
R. Yonah: Chachamim hearken to the words of their teachers. Even if they are not Mechadesh Chachmah via Tevunah, they store the Chachmah that they learned, lest they forget it. They sense what they do not know, and they are silent - "Gam Evvil Macharish Chacham Yechashev" (17:28).
Malbim: I explained above (1:2) that Chachmah is received. One cannot know it via proofs of Binah or senses. Even so, one can reach Da'as of Chachmah, if he accepted Chachmah as absolute Emes, guards it and contemplates it. Chachamim hide Da'as from people who damage, lest they turn it to evil. Also secrets of Elokus must be hidden; they are handed over only to individuals proper for them.
Here it says "Chachamim Yitzpenu Da'as.: Why does it say below "Arum Koseh Da'as" (12:23)?
Malbim: Tzofem does not totally cover the matter, just he puts it in a place where people cannot see it, but it is exposed. Lechasos (to cover) Da'as is the action of Armah, and not of Chachmah. They do not hide it from proper people - "Leshon Chachamim Teitiv Da'as", "Sifsei Chachamim Yezaru Da'as" (15:2, 7). Tzefinah includes guarding Da'as from improper people.
Why does it say "u'Fi Evvil Mechitah Kerovah"?
R. Yonah #1: An Evvil says everything that crosses his Ru'ach, even if it will cause damage via Rechilus, informing [to authorities], slander and evil counsel. His mouth is close to bringing demise to people. It was said, company of a lunatic is dangerous.
R. Yonah #2: If he boasts with his words amidst his anger, he will damage his friend. His mouth appears near, for he is prone to do all that he says. This is unlike "Pi Kesil Mechitah Lo" (refer to 18:7:1:1).
Malbim: An Evvil is one who is unsure about everything. He cannot have Da'as, i.e. clear knowledge, of anything. He hides and does not reveal even Mechitah Kerovah that will come soon, for he is unsure. Chachamim hide a good matter lest it be damaged; Evilim guard [and hide] open damage, lest it be revealed.
