Chevrolet 2006 TrailBlazer EXT Owner's Manual page 33

The safety belt locks if there is a sudden stop or a
crash, or if you pull the belt very quickly out of
the retractor.
CAUTION:
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You can be seriously hurt if your shoulder belt
is too loose. In a crash, you would move
forward too much, which could increase injury.
The shoulder belt should fit against your body.
To unlatch the belt, push the button on the buckle.
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