GMC C series 2002 Owner's Manual page 22

D
You can add a small amount of slack. The system
works like a window shade. To add a little slack,
pull down on the shoulder belt just a little
than 1 inch (25 mm).
D
If it's now too loose, pull it out like you did before
and start again.
D
If you move around in the vehicle and your shoulder
belt becomes loose, be sure to make it tight again.
The lap part of the belt should be worn low and snug on
the hips, just touching the thighs. In a crash, this applies
--
no more 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.
1-15