The pinball gear motor is mounted to the rear of the coin door and is operated by individual games, usually when a mechanical door, crane, lift etc is activated within a game, here are a few examples : 
 
The clock on Twilight Zone
T Rex head on Jurassic Park 
Crane on Dark Knight 
Slimers mechanical arm on Ghostbusters 
Dirty Harry's gun
 
Rather the hearing a computer generated gear sound the game will activate and turn on the gear motor giving you the more realistic feel and sound of mechanical gears !