good torsen discription, from

[Jay Rabe <brabe@snet.net>]

This was taken from
http://staff.connect.com.au/ianh/318ti/1998-01/msg00555.html
GLEASON (TORSEN) LSD:

TORSEN is a concatenation of 'Torque Sensing'.

This design uses a characteristic of worm and ring gears.  A worm gear
is usually a long thin helical gear that mates with the edge of a large
diameter ring gear.  When the worm gear is turned, it causes the ring
gear to rotate.  Tried the other way: attempting to turn the ring gear
directly will just cause everything to lock solid.  Motion is allowed in
one direction only.  This is partly due to the typical high gear ratio
involved with a conventional worm and ring gear pair, but a major factor
is the interaction of the shape of the gear teeth.

The actual gears inside the torsen don't have the appearance of
conventional worm and ring gears, but they still are classified as worms
and rings.  In this case, the worm gears are significantly larger in
diameter than the ring gears.

Each output shaft passes into the differential housing, where it is
driven by an output gear.  This output gear is a worm.

The differential housing has several pairs (3-5) of ring gears (they
don't actually look like rings) that are mounted in cutouts in the
housing.  Each pair of ring gears are connected to each other by
conventional gears that act as synchronizers.  One ring gear of the pair
meshes with the right output worm gear.  The other ring gear of the pair
meshes with the left output worm gear.  The reason several pairs (3-5)
of ring gears are used is to increase load capacity.

When the differential housing rotates, the ring gears are rotated around
the output worm gears.  Thus, force is being applied from a ring gear to
a worm gear.  As described above, no relative motion can occur between
these two gears because they lock up solid.  This means that full force
is being applied from the differential housing to the output shaft.
This occurs regardless of whether or not the other output shaft has any
load (traction).

When the car goes around a corner and one wheel needs to go faster, the
force from the faster outer wheel goes *into* the differential through
the output gear.  Now we have a situation where a force is being applied
from a worm gear to a ring gear.  Relative motion between these two
gears is allowed when the force is in this direction.

To summarize the two main characteristics in a different way:
Forces between the housing and an output shaft (engine power to a wheel)
are directly coupled.
Forces between two output shafts (differences in speed between the two
wheels) allow the internal gears to rotate.

The real beauty of this design is that these two characteristics are
autonomous.  Both things can be happening at the same time.  Full power
can be applied while going around a corner.  The wheels are allowed to
turn at independent speeds.  Full torque can be applied to a wheel even
if the other has lost traction. (Up to the equivalent of about 80% lock
up).  Changes in the situation are automatically adjusted for instantly
by the inherent nature of the design.  Everything operates in a precise
balance.

There is no need to choose a trade off between maximum traction, and the
ability to go around corners.

It's also important to note that while this design relies on the
friction characteristics of the gear teeth to control its behavior, it
*doesn't* use friction to transfer power (like a Clutch Plate LSD).
This design doesn't have any more wear than a conventional differential.

The Torsen is probably one of the most elegant mechanical designs in
automotive history.