GMC Sierra 2000 Owner's Manual page 29

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
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's a sudden stop or a crash.
1-16
Shoulder Belt Height Adjuster
Before you begin to drive, move the shoulder belt
adjuster to the height that is right for you.
To move it down, push in at the word PRESS and move
the height adjuster to the desired position. You can move
the adjuster up just by pushing up on the shoulder belt
guide. After you move the adjuster to where you want it,
try to move it down without pushing in to make sure it
has locked into position.