Contrary to popular belief, the electric opener does not provide the actual lifting power to open and close a heavy garage door. Instead, most of the actual lifting power comes from the counterbalance springs attached to the door. These springs are under tension to lift the garage door via steel counterbalance cables. The electric opener only controls how far the door opens and closes, as well as the force the garage door exerts. In most cases, the garage door opener also holds the door closed in place of a lock.
Information compliments of Wikipedia