In the event of a frontal impact of a certain severity, the airbag computer commands the pretensioner gas generator igniter.
Under the pressure of the formed gases, the piston of the cylinder is displaced, which moves the gear rack. This causes the gear and satellite gears to rotate. In this case, the belt is wound on a reel. Pretensioning the seat belt in the event of a collision reduces the risk of serious injury to front seat occupants from a sudden forward lean and impact against the steering wheel or instrument panel.