Some cars have just an airbag on the driver's side; others have both passenger and driver's side airbags. During a front collision of sufficient force, impact sensors close. This triggers an electrical signal that travels to the airbag module. Upon receipt of this signal, current arcs across both pins of an igniter and starts a chemical chain reaction. This arc ignites a canister of gas that ignites sodium azide pellets. The pellets burn rapidly and change to nitrogen gas. The rapid build-up of nitrogen fills the bag quickly. The rapid expansion of the airbag tears the steering wheel cover open at specific locations. The bag reaches full deployment shortly thereafter.
During a severe frontal collision, the airbag deploys and absorbs the forward force of the driver's or passenger's upper body as it spreads out over the large surface area of the bag. airbags and safety belts make a team. airbags don't replace belts. They only supplement them during severe front collisions.