What is winnowing? Well, after the grains have been removed from the stalks by threshing (in this case, by oxen), the grain is collected in a basket, the basket is raised, say, to shoulder height and tilted, and the grain is allowed to fall to the ground.

Winnowing separates the grain from the chaff. As the mixed grain and chaff fall from the basket, wind carries away the lighter chaff while the grain, which is heavier, falls into a pile on the ground.