As we apply a voltage between the top and bottom electrodes, the piezoelectric crystal expands. This pushes the tip closer to the sample. This would occur when the tip passes between atoms and the feedback circuit drives the tip closer to maintain the electron tunneling current.
To make it visible, here we are greatly exaggerating the expansion of the piezoelectric crystal. We have also added a virtual reality glow to show when the piezoelectric has expanded.
This arm provides what is called the Z, or Z-axis drive for the STM probe.
