Torsen, a contraction of "torque-sensing", was invented by an American company called Gleason Corporation.
The Torsen uses crossed axis helical gears to increase internal friction when one wheel spins faster than the other. The slower moving wheel always receives more torque than the faster moving wheel. The Torsen does not have a preload so some traction is necessary in order to transfer torque. If one wheel were raised in the air, the Torsen would act like an open diff and no torque would be transferred to the other wheel.
To learn more about how the Torsen works, visit Torsen.com
