You want to run into the airbag, you don't want the airbag to run into you. Airbags deploy in about 0.06 seconds at speeds of over 200 mph. They must be this quick because the typical automobile collision lasts about 0.125 seconds and unprotected occupants will hit the the vehicle's interior before the primary impact is over. That is, the second collision occurs while the first collision is still going on. Deployment is effected by a gas generator which usually employs the combustion of sodium azide to produce the volume of gas required.
Deployment is essentially the result of a small, contained explosion.
The sensor, or more likely, the sensor in conjunction with a computer, decides whether or not to deploy the bag based on the severity of the accident. The usual deployment algorithm involves in effect only the velocity change of the vehicle although more complicated algorithms are being incorporated into newer systems. Airbags first appeared in the early 1970's in GM and Ford test fleets, widespread introduction did not take place until Chrysler introduced them into its American made vehicles in 1989 as a result of government and consumer group pressure.