GMC Hybrid 2006 Owner's Manual page 18

Manual Override System
To manually override the automatic engine start/stop
system, press the tow/haul switch, located on the end
of the column shift lever.
2-10
This system disables the engine from performing the
auto stop function. While not required, you might choose
to use this during frequent start/stops, while driving on
grades or in heavy traffic.
Press the tow/haul switch again, or turn the ignition off, to
enable the auto stop function. The indicator light on the
cluster will go out.
When the tow/haul mode
has been activated, this
light will come on in
the instrument cluster.