The seat belt incorporating the pretensioner automatically winds the seatbelt upon front impact to reduce forward shifting of the passenger.
The seat belt pre-tensioner ignites the gas generator and emits gas with the electric current supplied from SRS-ECU. The gas pressure shifts the ball in the pipe and the balls comes in contact with the protrusion of the ring gear, which is inserted in the ring gear and then interlocked with the pinion is inserted in the ring gear The ring gear rotation forces the pinion to turn the spool toward the belt wind direction to wind the belt.