To "overcrank" means that the frame rate at which the film is being shot is higher than normal, so that when played back at a normal speed the action is in slow motion. Normal frame rate is 24 frames per second, so if you overcranked to 120 frames per second the action would be shown at 1/5 its actual speed. This shift can be intentionally obvious, to emphasize the dramaticnote details of a scene; it can also be used as a background effect — for example, in movies featuring Godzilla and similar monsters, overcranking adds a sense of ponderous weight to the monsters' motion. If someone's running, expect to hear the main theme from Chariots of Fire or The Six Million Dollar Man, at least in your head. This trope is also present in almost any shampoo commercial. Bullet Time is another form of overcranking, except in three dimensions.
Modern slowmotion footage is shot with digital cameras that have a very high framerate, rather than film, but the principle is the same, the footage is slowed down to 23-30 frames per second where it plays in slow motion.
Subtypes of Slow Motion include: