A material processing apparatus comprising a rotary operating device coupled to a material processing device, especially a crusher. A primary drive system is coupled to the rotary operating device. An auxiliary drive system has a rotary drive member and is operable between a driving state, in which it is coupled to the rotary operating device, and a non-driving state in which it does not rotate the rotary operating device. The apparatus is operable in a primary mode in which the primary drive system rotates the rotary operating device and the auxiliary drive system is in its non-driving state, or in an auxiliary mode in which the auxiliary drive system is in its driving state. In the auxiliary mode, the auxiliary drive system may be operated to repeatedly rotate the rotary operating device alternately in both rotational directions to impart a back-and-forth rocking motion to the material processing device.