Rotations occur upon each mouse click.
The step size here is the same as the 'large step size' in the subsequent dialog box. If you hold the mouse button down on while the cursor is on any of the rotation buttons of the toolbox, the image will continue to rotate until you let up on the button or position the cursor off one of the six rotation buttons. In' on a particular atom in the atomic structure. This is handy for the 'Hand Pick' and 'Draw Distance Line' options. Note that you MUST select an atom to zoom in on.