PREFIRE functions can reduce the risk of injury from head-on, side and rear crashes as well as vehicle rollovers. By tightening the reversible seat-belt tensioners, PREFIRE returns the vehicle's occupants in to their ideal seating positions before the crash and firmly holds them in place. This contributes towards an enhancement of the protective effects of the front and side airbags. A variety of signals can be used to trigger the activation of the seat-belt tensioners: ESC reports violent braking or critical lateral movement. The radar sensor of the ACC identifies the fact that the vehicle is approaching an object in front of it and the time before possible impact is calculated. The rollover sensor in the airbag control unit recognizes the danger of the vehicle overturning.