EDIT: I understand that it’s supposed to be for safety. I’m asking because I’ve only experienced the stopping mechanism when I pull. I’ve hit the brakes very abruptly before but nothing. Should I get it looked at?
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They stop when you pull them hard because the mechanism that makes the seatbelt lock up during a crash is (usually) a simple one that can’t differentiate. Basically, inside the mechanism there’s a U-shaped or J-shaped channel that belt runs through with a ball or roller inside, and the channel gets narrower near the top end. Normally, gravity keeps the ball near the bottom of the channel, which has lots of room, so the seatbelt slides right by when you pull it. But during a hard braking situation, the momentum forces the ball upwards into the narrow part of the channel, and the seatbelt sliding forward at that same time will lock it into place. However, because of the simplicity, if you pull the belt quickly, the ball won’t have time to roll, and will instead get pulled upwards into the channel, causing it to lock. This is also why you get issues in older cars, because those channels can get dirty over time.
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