Chevrolet 2003 Silverado Owner's Manual page 34

The safety belt locks if there's a sudden stop or a crash.
The safety belt also locks if you pull the belt very
quickly out of the retractor.
{
CAUTION:
You can be seriously hurt if your shoulder belt
is too loose. In a crash, you would move
forward too much, which could increase injury.
The shoulder belt should fit against your body.
1-28
To unlatch the belt, just push the button on the buckle.