Chevrolet 1998 Lumina Owner's Manual page 39

When the shoulder belt is pulled out all the way, it
will lock. If it does, let it go back all the way and
start again. If the belt is not long enough, see "Safety
Belt Extender" at the end of this section. 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 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 likel
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's a sudden stop or a crash
or if you pull the belt very quickly out of the retractor.
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