Same as jacking off, but in addition: Any time a mechanical device is temporarily malfunctioning, especially in an erratic motion fashion, it is beating off. See also on the rag and otr for malfunctions of a 5-7 day nature
Bill, the tachometer in you GTO is beating off. (wiggling erratically between 1400 and 3600 RPM)
when a girl puts a trashcan between her legs and rubs on it
She's beating off and laughing about it!!