They're a torque-splitting diff, they divide the power between the drive wheels. The one with the most traction, gets the most power.
It's basically a worm gear, a worm can turn a ring gear, but the ring gear can't turn the worm gear. Think of it like a one-way diff, except that it's not an all-or-nothing deal. In a one-way, all the power goes to the outside tire and the inside just goes along for the ride. In a Torsen, for example, you can have 70% go to the ouside and 30% to the inside.
In big cars, they're pretty much the best setup for road racing, very smooth power application and very reliable.