Here's an idea of how the Hummer's Crab Mode and four wheel steering system works.
The new system will have a narrower range of motion compared to the old GM Quadrasteer system. The old system could turn the wheels up to 15 degrees where the new system will only turn the wheels up to 10 degrees. Despite that it's still more than the range of systems like the ZF Active Kinematics Control.
For Crab Mode itself, the rear wheels turn in the same direction as the front wheels at low speeds. Aside from it being a cool party trick, it's supposed to increase off-road maneuverability to move diagonally around all kinds of surfaces.
This is from the Engineering Explained video: