Chevrolet 2009 Silverado Owner's Manual page 28

Sit up straight and always keep your feet on the floor in
front of you. The lap part of the belt should be worn low
and snug on the hips, just touching the thighs. In a crash,
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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 on 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 shoulder belt locks if there is a sudden stop or
crash.