Seat belts are an essential component of vehicle safety and all drivers and passengers in an automobile should wear them routinely. Car manufacturers designed the seat belt tensioner to help keep riders safe should an accident occur.
Seat belt tensioners keep the seat belt from fitting too loosely across your chest. The tensioner tightens the belt on impact to prevent you from jerking forward in a collision.
The seat belt tensioner locks in place if a sensor picks up that the driver is pressing too heavily on the brakes of the car. The seat belt then restricts your movement by keeping you firmly against the back of the seat.
Seat belt tensioners work in conjunction with the vehicle's airbag electrical wiring. They are part of a vehicle's Supplemental Restraint System (SRS).