Cadillac 2007 STS Owner's Manual page 22

6. To make the lap part tight, pull up on the
shoulder belt.
It may be necessary to pull stitching on the
safety belt through the latch plate to fully
tighten the lap belt on smaller occupants.
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7. This safety belt has a feature that will reduce
the tension of the safety belt on the occupant's
shoulder if the vehicle is on. To set this
feature, gently pull on the belt, or lean forward
and then sit back. The belt will retract and
rest lightly against the occupant.