The garage door opener uses a power unit that contains an electric motor. The motor uses a gear to drive a chain, belt or screw mechanism. These have a trolley connected to an arm, that attach to the garage door. The trolley is pulled along the track by a chain, belt, or screw. A quick-release mechanism is attached to the trolley to allow the garage door to be disconnected from the opener for manual operation during a power failure, or in case of emergency.
The garage door opener does not provide the actual lifting power to open a heavy garage door. Instead, the lifting power comes from the springs. These springs are under tension to generate torque to lift the garage door using metal cables. The garage door opener also controls how far the door opens and closes. In most cases, the garage door opener also acts like a lock when the door is close.