Overcrank

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 dramatic 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, at least in your head. This trope is also present in almost any shampoo commercial.

It's extremely easy to do this badly, or to use it as a parody.