Why does a combine blow the chaff out the rear while keeping the grain?
A combine blows chaff out the rear because wind is used to separate grain from chaff at harvest. The same concept as the ancient threshing floor, wind blows the chaff away while the grain is kept and goes up into the bin. They still use wind to separate it.
Les explains that harvest-time combines are the modern version of the threshing floor. The whole concept was that as grain was separated from chaff, wind was used to blow the chaff away. That is why, when watching a combine out in the field, the chaff blows out the rear while the grain goes up in the bin. They still use wind to separate it, and it has not changed. He connects this picture to the nations and great empires whose remains will blow away like the chaff of a threshing floor.