Chevrolet 2006 Express Van Owner's Manual page 22

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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