The rotating force is achieved by exciting the piezo elements on the stator in a pattern that generates a traveling wave. This wave makes the rotor rotate.
To get the maximum wave amplitude, and thereby maximum torque, the stator has to vibrate at its mechanical resonance frequency. This is around 45 kHz, but varies with ambient temperature and self-heating.
Therefore a tracking system to track the mechanical resonance frequency on the fly is necessary.
The output stage of the driver is designed to have an electrical resonance frequency close to the mechanical resonance frequency of the motor in order to increase motor efficiency. The electrical resonance frequency is defined by circuit design. Because of a low Q it does not need to be adjusted over temperature to track the mechanical resonance frequency.