How does seat belt allow movement at slow speed but disallow at high speed?

I have always wondered how seat belts work. If you try to move the seat belt at a low speed, there is nothing to prevent the movement, allowing adjusting the seat belt to a correct length. However, movement at a high speed is disallowed. There is some kind of mechanism that is able to detect the speed or the acceleration and prevent movement.How does the mechanism work? Does it detect speed or acceleration?What are the chances that the mechanism fails to work in a crash?...Read more