In an impact of sufficient severity, the airbag sensing system detects that the vehicle is in a crash. Additionally, in the case of a "rollover capable" roof-mounted side impact airbag, the sensing system detects that the vehicle is about to roll over. The sensing system triggers a release of gas from the inflator, which inflates the airbag.
For the frontal airbags, the inflator, airbag, and related hardware are all part of the airbag modules inside the steering wheel and in the instrument panel in front of the right front passenger. For vehicles with roof-mounted rollover airbags, the airbag modules, the inflator, and the airbags are located in the ceiling of the vehicle, near the side windows.