Crank arm and pin: the part that turns steadilyMaltese cross: a quarter turn per frame, then restA four-slot Geneva drive, the movement behind a film projector. The crank turns steadily; the cross snaps a quarter turn while the pin is in a slot, which moves the film one frame, then rests for three quarters of the turn while that frame is shown. Slowed about 30 times: at 24 frames a second, each turn takes about 42 ms.