GMC Yukon 2007 Owner's Manual page 45

6. To make the lap part tight, pull up on the
shoulder belt.
It may be necessary to pull stitching on the
safety belt through the latch plate to fully
tighten the lap belt on smaller occupants.
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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