GMC Yukon 2007 Owner's Manual page 56

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.
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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
a crash.
CAUTION:
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You can be seriously hurt if your shoulder
belt is too loose. In a crash, you would
move forward too much, which could
increase injury. The shoulder belt should
fit against your body.