Have you ever wondered how a Torsen differential works? By utilizing the clever principle that a spur gear (aka worm wheel) can rotate a worm gear but not vise versa, it enables simple, gear based differential locking that’s always available, and never needs to be turned on or off. Torsen differentials are used as center diffs in Audis, and as front and rear diffs in Humvees.

Links:
Wikipedia