How does an airbag work?

Question

I know that some sort of explosion occurs at some point but I don't know how it knows when to do explode or what sort of explosion it is.

Answer 1

An airbag system consists of the airbag module which contains an inflator, the airbag itself, and a trim cover. The proper firing of the airbag is controlled by crash sensors which supply electrical signals to a control unit which fires the inflator to inflate the airbag.

In the 15 to 20 milliseconds after impact, the crash sensors and control unit determine the severity of the collision and decide whether to deploy the airbag.

At about 25 milliseconds, the airbag splits its covering pad in predetermined places and begins to inflate rapidly.

At about 45 milliseconds, the bag is fully inflated while the seatbelted occupant is still moving forward.

At around 60 milliseconds, the occupant contact the airbag which immediately begins to deflate via vent holes in the back.

Up to 100 milliseconds, the occupant continues to sink deeply into the airbag which cushions the head and chest while it is deflating.

In less than a blink of an eye, the crash has occurred, the airbag has deployed, done its job and now lies deflated in front of you.

Answer 2

A mixture sodium azide and potassium nitrate react after the igniter (small rocket booster) is triggered to produce hot nitrogen gas which is shot into a nylon bag at a rate of about 200mph which happens in about one-twenty-fifth of a second.  It deflates quickly because of the two 2" or so holes at the base of the bag.