Movie 1.

This video shows two views of the same spermatozoon at two different focal planes separated by 2 μm. In the left panel, the determination of the head rotating direction is not possible, while in the right panel, the bright flash moving from left-to-right gives the information to infer the rotating direction of the cell (note that this bright flash is located behind the translucent head, indicating a CCW rotating direction, seen from head-to-tail).