Torsens work using helical(worm) gears. What makes the diff work is that worm gears only work, and not work, in one direction each, that is also the downside. Think of the usual use of a work gear, to give huge amounts of gear reduction, in a set that can only be turned from one end. You turn the shaft the worm gear is on, driving a spur gear at a much lower speed. If you try to turn the shaft the spur gear is on, you can't. In a Torsen diff, power from the engine is basically trying drive a set of worm gears by turning their mating gears, but working backwards, under a braking load, those those worm gears a driving instead of being driven, so in that situation it work like an open diff.
As i've been able to wrap my head around, that's the basic gist of it. I'm no engineer, just a generally mechanically minded bloke that likes to take stuff apart to see how it works.
*EDIT* Heres a page that give a better explanation...and a lego torsen model
http://individual.utoronto.ca/stehlik/torsen.html