How a Seatbelt Works
A seatbelt pretensioner activates when it senses the vehicle decelerate abruptly. An explosive charge moves a piston, the piston spins a spool around which the seatbelt fabric is wrapped. This tightens the belt to limit movement during an accident.
We can see how the mechanism works in this video clip, thanks to the Slo-Mo Guys YouTube channel.
Red paint was added for visual effect.
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