Seat belts must be worn tightly so they can restrain occupants quickly in the event of an accident. But because of clothing and the soft inertia reel mechanism, this is not always the case.. Pyrotechnic belt pre-tensioners compensate for this by tightening the belt on impact. They are triggered by a gas generator via the airbag control unit. The belt tensioner has a lower trigger threshold than the front airbag, meaning it may trigger in response to a collision which is not serious enough to activate the airbag. When triggered, the tensioners pre-tension the seatbelt. As the seatbelt is then held tighter to the body, it reduces the risk of injury in the event of an accident.