What physical phenomenon is observed when threshing grain with a combine drum?

it is inertia
When the grain is threshed with a drum in the combine, a physical phenomenon of inertia is observed.
Inertia is the property of the body to maintain its state. The grains in the ears are at rest, under the action of the drum, the ears are set in motion, and the grains in them retain their resting position. Separation of grains and ears occurs. When ears with grains and under the action of the drum begin to shake, then due to inertia the grains retain their resting position, and the ears begin to move. The separation of the grains from the ears occurs.