The sample material is primarily crushed by the high-energy impact of grinding balls together with friction between the balls and the wall of the grinding bowl.
The grinding bowls with material and balls rotate around their own axis on a counter-rotating supporting disc. The centrifugal forces caused by the rotation of the grinding bowls and supporting discs work on the contents of the grinding bowls. The force resulting from rotation of the grinding bowl when the mill is started causes the rotating balls to rub against the inside wall of the bowl thus crushing the material.
At a certain point in time the stronger centrifugal force of the supporting disc causes the grinding material and balls to separate from the inner wall of the grinding bowl. The grinding balls cross the bowl at high speeds impacting with the grinding material on the opposite wall - creating size reduction by impact.