A seatbelt will let you lean all the way over to the glovebox, and it will not let you go six inches in a crash. Nothing in it knows which is which. Inside the retractor there is usually a small weighted pendulum that swings when the car itself lurches, and a weighted lever on the spool that flies outward when the belt is pulled too fast; either one tips a pawl into a toothed wheel and the spool stops. So the whole judgement is made on speed. Reach slowly and you are a person getting a map. Move suddenly and you are something the belt has decided to keep. It is a fair rule for a machine, and you can test it yourself in a parked car: tug the belt sharply and it catches, then ease it out and it lets you have as much as you like.