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In certain situations, an open diff can be made to behave like a limited slip diff by applying the brakes to the spinning wheel. That is because an open diff will always send torque to the wheel with less traction and spin it. So by braking the spinning wheel, TQ gets sent to the wheel with more traction, electronically simulating a limited slip diff (LSD). Xdrive uses this feature on both the front and rear axles. RWD 335 cars have a similar feature for the rear diff only. It is not an optimal solution, a conventional LSD is better. M3 has a true LSD, and I think X6 gets an xDrive with true LSD ("torque sensing").
Xdrive has three layers of software, DSC, DTC, and then both off. DTC is a subset of DSC, lighter stability control and no engine power reduction, but traction control is still on though a bit more agressive, allowing a bit of oversteer. With DTC/DSC off completely, in xDrive you still have the front and rear e-LSD (via brakes) and the transfer case working. So IN THEORY as far as I can tell, with DTC and DSC off you may be able to spin all four tires on ice. Have not tested this yet as I have not had the car in winter.
As I have said in other threads, BMW is very cagey about how all this works.
BTW ABS and "e-LSD" both work through the brakes of course, but one works by modulating (cutting) pressure in the lines on lock up, the other actively applies the brakes based on sensors and electronics to stop wheel spin. So in theory they are totally separate systems, both of which act on brakes but no need for them to be in the same fuse circuit or even same control algorithm. They have nothing to do with one another as one works when you apply the brakes the other is when you give it gas. Then there's stability control which another system entirely. They all talk to one another of course.
I personally would not want to yank a fuse out of the middle of all this, disable ABS and all layers of DTC/DSC/e-LSD, and then push around dead rotating mass of front AWD system. But I admire the spirit of experimentation it represents. Proper solution is for Shiv or someone similar to do this w/o messing with the other sytems (if that even can be done).
edit: the above is as far as I have been able to figure out, reading a lot here and on other forums. Anyone has any knowledge to the contrary please chime in.
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