To help increase passenger protection, the Pretensioner reduces the slack between the seat belt and the passenger’s body during a collision. When a collision occurs, the pretensioner is activated and the power generated by the micro gas generator instantly recoils some webbing back into its housing. This enhances the effectiveness of the seatbelt by bringing the passenger as close to the seat as possible.
The sequence for this is as follows: The signal from the sensors is transmitted to the ECU (electronic control unit) which determines if a collision has occurred. If this is the case, a signal is sent to the pretensioners in the seat-belt system initiating the webbing retraction. The Pretensioner in the belt almost instantaneously winds webbing back into its housing, removing all slack.
Pretensioners may be installed in various places. In some vehicles, the Pretensioner is in the retractor. In other vehicles, the Pretensioner is part of the buckle assembly. Additionally, some vehicle may have both.