When you are travelling in a car, you and the car are moving at the same speed. If the car stops abruptly, your body keeps moving forward. This is an illustration of inertia – the tendency of a moving object to keep moving, or of a stationary object to remain at rest.
An inertia-reel seat belt works on the same principle. Its mechanism includes a pendulum, which hangs vertically under ordinary driving conditions. But if the car stops abruptly it swings forward, and a locking lever resting on the pendulum is released. The lever engages a toothed ratchet that locks the shaft round which the belt is wound. The locked seat belt then prevents your body from being flung forward.
When you fasten a seat belt, it winds out from the reel against slight tension from a spring. This keeps it taut during normal travelling, but allows enough free movement for a driver to reach forward as necessary. But if you tug abruptly on the belt while winding it out, the locking mechanism will engage and stop the action of the spring. Slackening the belt releases the spring and the locking lever.
Picture Credit : Google