GMC 2009 Sierra - Manual (Page 20)

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, 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.
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