Well, you think that you understand something and then you come to find you're wrong again...
I found a video on YouTube that shows how the Eaton TruTrac differential functions to distribute torque. It is another helical-gear design, but only uses 3 helical-planetary gears per side (they call them "pinions"). The essence of the design is the same as Quaife, OBX and the Type-2 Torsen. And it all comes down to this - The planetary gears don't rotate when the torque bias is equal between the two sides (50%:50%). Once the ratio changes, the planetary gears begin to rotate. And though the planetary gears for both axles mesh in the center of the differential (common to all designs mentioned), they somehow allow the axles to rotate at different rates... which appears to be all based on torque and the pitch of the helix (changing the pitch apparently changes the torque-bias ratio [TBR]). So it might not really involve "slip" - at least not in the same sense as you find in a clutch-based diff.
Also, I don't think the spring-effect of the belleville washers is critical to the function of torque-distribution. After doing a bit more research, I came across some posts that claim the first Quaife diffs didn't have the spring-stack. They were added to eliminate the annoying sounds caused by the sun gears rattling in the case during turns. Again, it is important to orient the sun gears so they thrust away from the spring assembly "naturally", when under the load created by forward travel.
Here's a link to the clip on YouTube... The demonstration of the torque-split starts around 1:20 min in the video.
Eaton TruTrac video Here's another clip that shows how the T1 torsen works...
T1 Torsen clip Oh, you can't leave out the Lego model of the T1 Torsen...
Lego Model of T1 Torsen... And here's a link to another clip that shows how the TruTrac distributes torque - the Land Rover shown has the TruTracs installed in the front and rear. The "touch the brake" technique is shown at the very end...
Land Rover with TruTracs demo
