Stabili Track and Traction Control

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Short version.
Traction Control:
When the electronic brake control module (EBCM) detects one or more of the drive wheels slipping, and the brakes are NOT applied the EBCM will send a pulse modulated signal to the PCM. (If brakes are applied it goes into ABS braking.)
The PCM will reduce torque to the drive wheels, by retarding timing, and it will also take over control of the throttle plate. The PCM in turn will signal the EBCM with the amount of torque being applied to the drive wheels.
If the reduction in torque does not work, the EBCM will pulse the brake at the wheel that is slipping.

Active handling:
Using a yaw sensor to detect the amount of yaw in the vehicle (yaw=rate of rotation on the vertical axis). The yaw sensor also detects the sideways acceleration of the vehicle with a built in lateral accelerometer, and the EBCM monitors this lateral acceleration.
If the EBCM detects that the programmed desired yaw rate and lateral force do not match the internal calibration, it will use differential braking to pulse the brakes.
For example if your rear end is sliding to the right, it will pulse the front left brake, then pulse any other brake require to pull you into a straight ahead direction.
It also relies on inputs from a steering wheel sensor to determine the position of the steering wheel, the speed of the vehicle and master brake cylinder pressure.

When in active handling, the brake pedal pulsations will feel a bit different then when ABS brakes are being applied. This is due to a different rate and frequency being pulsed by the EBCM.

The stability control will help to prevent a roll over.

I have never gone into active handling with the Hummer, but on a Vette it works wonders and prevented someone from going over a cliff in NC (won't say who).