GMC TopKick 2007 Owner's Manual page 28

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 would 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.
The safety belt locks if there is a sudden stop
or crash.
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