There's two ways grain can exit the back of the machine. One way is for grain to go over the chaffer, just like on a conventional machine. The other way would be for grain to be mixed with the straw that goes out the rear end of the rotor. From there it goes to the straw chopper (or beater for machines without a chopper) and then back to the straw spreaders. There is no way to capture the grain that exits the rear of the rotor, which is called rotor loss. In order to save the grain, separation must be completed by the time the crop gets to the rear of the rotor.