Seat belt mechanism - how does it lock?

[Barbecue]

Hey guys,

I have a quick question about the seat belts in my '86 Silverado. From what I understand, the seat belts on our trucks the same 1978 and up. The seat belts I have are the single box units beside the bench (retractable shoulder belts).

My question is, if I'm in an accident, what mechanism (what action per se) actually locks the shoulder belt?

The reason I ask this is that with other older vehicles I've worked on, if you yank on the shoulder belt, it locks. This is basically simulating a person's torso lurching forward in an accident. When I yank on the shoulder belts in my truck, the belt do not lock. The belt themselves do retract, albeit a little weakly. Is there some other kind of mechanism, perhaps a pendulum or something, which locks the shoulder belt in the event of a crash?

Thanks in advance for any insight you can offer. I'd just like to have the confidence knowing that my passengers (and I) are safe.

BBQ