Buick 2005 LeSabre Owner's Manual page 30

The safety belt locks if there is 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.
1-24
To unlatch the belt, just push the button on the buckle.