“A wise king separates out the wicked, then he rolls the threshing wheel over them.” Anyone who lived in the culture of the Old Testament would realize at once that grain was harvested and gathered, then it was threshed, then it was winnowed (which was to separate the wheat from the chaff), so this verse at first glance could be thought to be backward, but that is not the case at all. Instead, there is a very profound meaning in the verse.
In the culture of the times, at harvest the grain was cut, and then placed in huge piles on the threshing floor. Then a threshing instrument was applied to the grain so that the heads of grain were separated from the stalk. The threshing instrument could be as simple as a stick that pounded the grain, or an animal could be led back and forth over the grain (hence the saying, “Do not muzzle the ox that treads out the grain”), or a “threshing sled” or cart could be dragged or rolled over the grain. Once the grain was threshed and separated from the stalks, the mixed piles of stalks and grain were winnowed during a light wind. The winnowing was done by throwing the mixture of stalk and grain high into the air. The wind carried the stalks to the side of the threshing floor, and the chaff, the small pieces of broken stalk, even further to the side, but the small round grain fell more straight down.
As the winnowing was done over and over, eventually only mostly grain would be left, which then had to be sifted in a grain sieve. That would normally end the process, and the grain would be ground for flour. But in this proverb, the wicked are compared to grain that the king winnows (implying he has already threshed it), but the king is not satisfied. He believes there is still some wickedness left in his kingdom and brings the threshing wheel over them again. That the threshing wheel “returns” over the wicked is not well understood, or well represented in most versions, which makes the verse confusing and backward. The point of the parable is that wise rulers (and thus also wise people) make sure that wicked people are removed from their kingdom, their business, or whatever they are doing, and that means going over and over the people present to weed out evil.