GMC 2001 Sierra - Manual (Page 52)

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The safety belt locks if there’s a sudden stop or a crash.
The safety belt also locks if you pull the belt very
quickly out of the retractor.
CAUTION:
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, just push the button on the buckle.