An air bag is inflated to protect an occupant of a vehicle upon the occurrence of a vehicle collision.  When the vehicle experiences a collision-indicating condition of at least a predetermined threshold level, an inflator is actuated.  The inflator then emits inflation fluid which is directed to flow into the air bag.  The inflation fluid inflates the air bag to an inflated condition in which the air bag extends into the vehicle occupant compartment.  When the air bag is inflated into the vehicle occupant compartment, it restrains an occupant of the vehicle from forcefully striking parts of the vehicle as a result of the collision. 
The air bag and the inflator are typically assembled together as parts of an air bag module.  In addition to the air bag and the inflator, the module includes a reaction canister which contains and supports the air bag and the inflator in the vehicle.  The reaction canister has a deployment opening through which the air bag moves outward from the reaction canister when the air bag is inflated.  A deployment door extends over the deployment opening to conceal the air bag and the other parts of the module from the vehicle occupant compartment. 
When the inflator is actuated, the reaction canister directs the inflation fluid to flow from the inflator into the air bag.  As the inflation fluid enters the air bag, it moves the air bag outward through the deployment opening and forcefully against the deployment door.  The deployment door is ruptured by the force of the fluid pressure in the air bag, and is thus released for movement away from the deployment opening.  As the air bag continues to move outward against the deployment door, it forcefully deflects a hinge portion of the deployment door so as to move the deployment door pivotally away from the deployment opening.  The deployment door is thus opened and moved pivotally out of the path of the air bag as the air bag inflates outward from the reaction canister through the deployment opening and into the vehicle occupant compartment.