GMC Denali 2004 Owner's Manual page 32

The lap part of the belt should be worn low and snug on
the hips, just touching the thighs. In a crash, this
applies force to the strong pelvic bones. And you'd be
less likely to slide under the lap belt. If you slid under it,
the belt would apply force at your abdomen. This
could cause serious or even fatal injuries. The shoulder
belt should go over the shoulder and across the
chest. These parts of the body are best able to take belt
restraining forces.
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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.
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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.