I believe the Torsen was originally called the Dual Drive many years ago. It's principal is that a worm gear can turn a worm wheel but a worm wheel cannot turn a worm gear.
Basically, it works by allowing one of the wheels to turn faster as long as it's an external force that's doing the driving, like the outside wheel in a corner. The inside wheel can drive slower only to the extent that the outside wheel is overdriving. In a case like this, torque is applied to both wheels equally.
If one wheel is off the ground and you try to go forward, the wheel without traction cannot drive faster than the wheel with traction because the no traction wheel is not being driven by an outside force. It is being driven by inside force (engine).
This type of differential is never 100% locked because their are compromises made to the angle on the gears. But it is an excellent differential for a road car because it allows equal torque to be applied to both wheels in a corner unlike friction clutches which need to slip to allow the difference, or a detroit locker which always drives the slower turning wheel.

Sorry for the wordy explaination.