Seatbelts have locking mechanisms that prevent the belts from slackening, which keeps the driver and passengers from falling too far forward in a collision, according to Takata Corporation, a seatbelt manufacturer. The locking mechanisms engage when the car's speed suddenly slows.
If a car is involved in a collision, the seatbelts slacken a small amount and lock as the airbags deploy. This allows passengers to fall slightly forward, letting the airbags absorb the weight of the impact. The locking of the seatbelts and the airbag deployment take only about one-tenth of one second to complete once the vehicle's sensors detect a collision or a decrease in speed, according to Takata.