GMC 2007 Acadia Owner's Manual page 42

The safety belt locks if there is a sudden stop or
a crash.
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.
42
To unlatch the belt, push the button on the buckle.
For outboard seating positions, when the safety
belt is not in use, slide the latch plate up the safety
belt webbing. The latch plate should rest on the
stitching on the safety belt, near the guide loop on
the side wall.