Pressure applied by the piezoelectric stack on the diaphragm causes it to shear,
which in turn drives the tuning fork's tines apart from each other. See the figure given aside for a
simulation of the process. When the pressure is removed the fork tines return to their original
position.
The application of force when applied at the fork's Natural Frequency makes
it oscillate at its maximum amplitude for a given power.
Amplitude Change in Vibrating Fork for Solids