Arming the emergency escape slides causes a metal girt bar attached to the girt at the upper end of each slide to be connected to the floor of the airplane cabin. In the event of a situation requiring emergency evacuation of the passengers from the airplane, current Federal Aviation Regulations require that each emergency escape slide be deployed by moving the door handle connected to the inside of the cabin door to the open position. This causes (i) the cabin door to open (on some airplanes the door must be manually pushed open), and (ii) the uninflated escape slide to fall from its storage pack inside the cabin door and to begin inflating outside the airplane. Since the upper end of the inflating escape slide is attached to the girt bar, the escape slide remains connected to the airplane.

Source: http://www.google.com/patents/US5400985