I just read this on how stuff works and they actually got it from a Hummer manual. We all know that in low traction situations like ice or snow the torsen acts as an open differential. If you use the parking brake to increase the resistance of the wheel that is on ice, the differential will start to react and produce torque on the opposite wheel! I wouldn't recomend doing this for more then a few seconds to avoid damage to the parking brake, but it could really help someone out of a slick spot.

p.s. I wouldn't recomend using the regular brake because the front wheels would hold you in place even if both wheels did start to spin. We're not trying to do burnouts here!