What is thickness-shear mode technology?
They're pretty much what they sound like. Piezo resonators that operate in the shear mode. Obviously that mode of operation is much, much higher frequency than bending modes (MHz). 

If the resonator is a disk, you're pushing (shearing) the top relative to the bottom without bending. 

They're useful for Bulk Acoustic Wave (BAW) viscosity sensors and other types of instrumentation.