Like deep, still waters, so the hearts of men can deeply conceal their plans. The word "plan" here is key to understanding this particular proverb. What this word means will govern what we need to learn. The word is "esah" and it means advice or a plan. What helps us most in understanding this word is the comments of Zhodiates who says the following about this word. "It sometimes suggests the idea of a plot."
The plots and plans of wicked men are hidden deep in still waters deep within their hearts. Isaiah speaks of these kinds of hidden plans and plots when he says, "Woe to those who deeply hide their plans from the LORD, And whose deeds are done in a dark place, And they say, 'Who sees us?' or 'Who knows us?'" (Isaiah 29:15). Thus we come to understand that this proverb is warning us of the ungodly plots of the wicked. We are told that these plans exist hidden deep within the hearts of the wicked - but a man of understanding will draw them out.
This ability belongs only to the "man of understanding." He is the one who possesses the ability to distinguish between the good and bad, true and false, and the forthright and the one whose motives are deeply hidden. This man of understanding has the capacity to look through outward actions and words, and see the hidden places of the human heart. This one does not accept everything at face value alone. He not only estimates their words, but the deep underlying motives and aims that are behind them. It is such wisdom that allows the man of understanding to discern hidden motives or hidden agendas. This is the way that he draws out the hidden plans and plots of the wicked.
In the New Testament this Spirit-given ability is called the gift of discernment - or discernment of spirits. It is a wonderful gift God has given to some to see below the surface. They are able to see into the heart by God's grace. What they see allows them to read below those deep waters - and see the real reason something is being said or done. Normally, without the Spirit of God, someone could easily pull the wool over our eyes and deceive us. A plan might be laid that we think is gracious, but actually has harm in store for us. That is when the ability to draw out the plans and plots of men is such a blessing.