An inflatable vehicle occupant protection device, such as an air bag, is part of an apparatus which further includes a collision sensor and an inflator.  When the collision sensor senses a vehicle collision having at least a predetermined threshold level of severity, the inflator is actuated.  The inflator then emits inflation fluid which inflates the air bag into the vehicle occupant compartment to help protect an occupant of the vehicle. 
An air bag and an inflator are typically assembled together as parts of an air bag module which is separate from the collision sensor.  The air bag module is located in the vehicle adjacent to the vehicle occupant compartment.  For example, an air bag module may be located on the vehicle steering column or in the vehicle instrument panel.  An air bag module may also be located in a side portion of the vehicle, such as a door panel, or in another part of the vehicle, such as the roof, the floor, or a seat. 
In addition to the air bag and the inflator, an air bag module may include a diffuser and a cover.  The diffuser is located between the inflator and the air bag.  The cover conceals the other parts of the module from the vehicle occupant compartment, and includes at least one pivotal deployment door which extends directly over the air bag. 
When the inflator is actuated, the inflation fluid emitted from the inflator flows into the air bag through the diffuser.  As the inflation fluid begins to inflate the air bag, it moves the air bag forcefully outward against the deployment door on the cover.  The force of the fluid pressure in the air bag moves the deployment door pivotally open as the inflation fluid continues to inflate the air bag outward from the air bag module into the vehicle occupant compartment.