from Wiktionary, Creative Commons Attribution/Share-Alike License
- v. To shoot a scene while advancing the film in the camera more slowly than it will on screen. This results in speeding up the action in the final film.
From under + crank; dating from a time when movie film was advanced in a camera by the turning of a crank. (Wiktionary)
Many high end cameras have overcrank and undercrank with variable frame rates in increments/decrements of 1