[pmpski_1]

I had the cover off mine recently and saw how they work. The belt is on a roll, and below it is a weighted thing that swings freely. (I don't know exactly how to describe it, but I'm sure there's a name for it). When you stop hard enough, the bottom of the weight swings and wedges the top against the roll, causing it to lock up.

Hope you get the idea from my cruddy description.