GMC 2002 Yukon Denali Owner's Manual page 96

Child Security Locks
You will find a security lock lever on the inside edge of
each rear door. Your vehicle may be equipped with one
of the two labels shown.
2-8
With this feature, you can lock the rear doors so they
cannot be opened from the inside by passengers. To use
the security locks do the following:
1. Open one of the rear doors.
2. Move the lever up to engage the security lock.
Move the lever down to disengage the security lock.
3. Close the door.
4. Do the same thing to the other rear door.
The rear doors on your vehicle cannot be opened from
the inside when this feature is in use. If you want to
open the rear door when the security lock is on,
unlock the door and open the door from the outside.