GMC SIERRA 1997 Manual page 44

3. To make the lap part tight, pull down on the buckle
end of the belt as you pull up on the shoulder part.
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 like
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 g
over the shoulder and across the chest. These parts of th
body are best able to take belt restraining forces.
The safety belt locks if there's a sudden stop or a crash.
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