Why does chaff blow out the rear of a combine while the grain goes into the bin?
Wind is used to separate the grain from the chaff. In a combine, that wind blows the chaff out the rear while the grain is collected and carried up into the bin. This is the same threshing-floor principle, and it has not changed.
Les explains that most people have seen wheat country at harvest time, and today the huge combines still picture the ancient threshing floor. The whole concept is that as the grain is separated from the chaff, wind is used to blow the chaff away. That is why, when you watch a combine in the field, the chaff blows out the rear while the grain goes up into the bin. They still use wind to separate it, unchanged. Les then applies this image to the nations and great empires whose remains will blow away like the chaff of a threshing floor.