By cycling the voltage across the piezo, it repeatedly stretches and contracts in the vertical direction. This cycling occurs so quickly (e.g., 200 MHz) that the whole tripod does not have enough time to move. Instead, its black plastic frame just stretches and compacts to accommodate the piezo’s vibrations.
But the cantilever assembly is mounted just below the vibrating piezo. So the piezo's vibration also causes the cantilever to swing up and down.
The reflected laser beam (bouncing off the cantilever arm) then also oscillates backward and forward across the face of the photodetector. But with no sample in place, the reflected laser beam still hits the photodetector forward of the yellow arrow.
