Hummer 2003 H2 Owner's Manual page 122

To use this feature, ensure that the following conditions
are met:
D
Be sure all the doors are fully closed. The suspension
will not raise or lower if a door is open.
D
Be sure the vehicle speed is less than 35 mph
(56 km/h). The system will not activate otherwise.
Press the height control button to raise the rear of
the vehicle.
A light in the button will begin to flash as the rear of the
vehicle rises. Once the extended height has been reached
(this may take up to a minute), the warning light will
stop flashing and will stay lit while the vehicle is at the
extended height.
2-40
To lower the vehicle to the normal ride height press the
button again. The light in the button will flash as the
suspension lowers. When the light in the button stops
flashing and goes out, the suspension has reached the
normal ride height.
This feature will lower the vehicle to the normal height
if vehicle speed exceeds 20 mph (32 km/h).
You may also want to use this feature when ascending,
descending or cresting a steep hill as this can help
prevent the rear bumper from dragging on the base
of the hill or prevent the vehicle from grounding out
(high centering) on the crest of the hill.
Do not use this feature when towing a trailer.