This is what I could find, basically a computer decides when its needed. The driver is not in control of anything anymore. LOL
Electronic Limited Slip Differential (eLSD)
All-Wheel Drive XTS models are equipped with an electronically controlled limited slip differential (eLSD)in the rear axle, ensuring optimum traction and handling performance in a variety of road conditions, including snow, mud, sand or gravel. When available traction is low, this feature allows the wheel with the most traction to move the vehicle. It also enhances driver control of the vehicle when cornering hard or completing a maneuver, such as a lane change.
and this description
http://auto.howstuffworks.com/elsd2.htm
