You know seat belts are affected by gravity right? You need to hold the 'seat belt unit' upright like it would be mounted in a car THEN pull it. There's a ball bearing or something like that in there locks seat belts when the car's on its roof.
Totally off topic here but sometimes you can tell a car has been in a decent hit and fixed not good by the seat belts. One car I rode in a few times, the seat belts units would jam when you're on a hill and won't let you put the seat belt on until you're back on level ground. The result of a decent hit to the b-pillar. Try not to buy cars like this.