The seat belt must restrain the occupants as quickly as possible in the event of an accident, and so must be worn tightly. Because of the occupants' clothing and the soft inertia reel mechanism, this is not always the case. This effect is termed belt slack. Pyrotechnic belt tensioners compensate for belt slack by tightening the belt on impact. Belt tensioners are triggered by a gas generator by way of the airbag control unit. The belt tensioner has a lower triggering threshold than the front airbag, meaning it may trigger in response to a collision which is not serious enough to trigger the front airbag. When triggered, the tensioners pre-tension the belt strap. As the belt is then tighter to the body, the occupants decelerate along with the vehicle at an earlier stage, and the loading on the body is distributed evenly across the full restraint process, thereby reducing the risk of injury.