Rotary models can be ball, roller or turbine action. In the former model a ball is rotated by air pressure around the inside of a hardened steel race to give an out of balance force. The speed of rotation and the force output are varied by changing the inlet air pressure. This principle also applies to the roller type. In the turbine, an out-of-balance turbine is held in bearings while being rotated. In this case there is no metal to metal contact which gives a much quieter operation with no lubrication required.
Again, varying the air inlet pressure will change the frequency and force.