Lexus GS430 User Manual page 209

OCCUPANT RESTRAINT SYSTEMS
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2. Fully extend the shoulder belt to put it in the lock mode.
When the belt is then retracted even slightly, it cannot be
extended.
To hold the convertible seat securely, make sure the belt is in
the lock mode before letting the belt retract.
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3. While pressing the convertible seat firmly against the seat
cushion and seatback, let the shoulder belt retract as far as it
will go to hold the convertible seat securely.