Buick Century 2002 Owner's Manual page 38

The safety belt locks if there's a sudden stop or a crash,
or 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.
1-32
To unlatch the belt, just push the button on the buckle.