 If a rapid evacuation is required and the doors are opened while "armed", the opening of the door pulls the slide pack out of the bustle (because the girt bar is physically attached to the aircraft floor).Due to the weight of both the door and the slide, great effort is involved in pushing the door open sufficiently to free the slide from the bustle, thus on larger aircraft a "power assist" function kicks in to aid the opening, either electrically or from compressed gas.Once the slide is completely free it will fall under gravity and after travelling a certain distance a pin will be pulled from a squib containing compressed gas and the slide will inflate.