Eli5: How/why do seatbelt lock and why does pulling it back unlock it?
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This is the point of a seatbelt – if the passenger is moving forward quickly and suddenly then they need to be restrained. Furthermore this should be its own mechanism, not activated by any kind of “crash detection” hardware anywhere else in the car.
Generally they work by having a spinning mechanism as the belt is being pulled. As a thing spins, centripetal force makes the parts inside move from the center to the edges. Springs try to hold the parts in the middle, but the faster the belt is pulled the harder the parts are pulled to the edges. If they actually touch the edges, they catch on teeth on the edges and jam the motion, preventing any more belt from spooling out. Let the belt move backwards a bit to release the mechanism’s bite and springs pull it back towards the center. Now you’re back to normal mode.
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