Chevrolet Impala 2007 Impala Owner's Manual page 38

If the belt is not long enough, see Safety Belt
Extender on page 42.
Make sure the release button on the buckle is
positioned so you would be able to unbuckle
the safety belt quickly if you ever had to.
3. To make the lap part tight, pull up on the
shoulder part.
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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
a crash.