A proper seat belt when pulled suddenly should stop immediately and when pulled gently should come out smoothly. In case of sudden stop or collision these belts lock and hold the driver/passenger in the seat and prevent them from banging their heads on the windshield or hitting steering or seats in the case of rear passengers.
I am angry at people not wearing seat belts especially the rear ones available except in Cowrolla XLi. Even people traveling with me when refuse to wear seat belt, I told them please wear it because I do not want them to break my windshield with their head in case of an accident. But still they don't and say that no worries you will not get a ticket for it. Damn. I constantly preaching benefits of wearing a seat belt and sometime people traveling with me do understand and pull out the belt......but still a colleague who always travel on the front seat when we are on assignments and also in the habit of sleeping during travel do not wear seat belt despite my narrating horrible experiences of people.
Seat belts do save lives and prevent serious injuries.