Buick 2007 Rendezvous Owner's Manual page 50

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.
50
To unlatch the belt, push the button on the buckle.