It is possible to make the mechanisms a few different ways. But the most common ones have a few weights on the spinning axle. Normally these weights are close to the center of the axle due to a few small springs. However when the axle moves fast the weights gets forced out through the centrifugal effect. As the weight move out they catch some catches on the housing of the unit and therefore locking the entire mechanism from spinning. This way you are able to slowly let the seatbelt out when you are getting into the car and when you move around in the seat. However in the event of a collision the seatbelts will move too fast and lock in place.
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