I'll probably get flamed for this...
It's impossible to lose complete traction (yes, even ice has some "traction") in real world driving such as the test on the rollers aside from the tire being completely up in the air. Therefore, these tests on youtube are irrelevant to a prospective buyer who will use this "AWD" system in on-road situations 99% of the time.
Torsen-based Quattro is a mechanical based system which will require driver intervention when the tires are on rollers. If you are curious how the system operates, Wikipedia it and it will explain how Torsen functions and that it needs some resistance for torque transfer, otherwise it acts like an open differential. In a wheels on roller or wheels in air situation, the driver either needs to:
1) step on the brake at the same time to create this resistance within the drivetrain
2) use the E-brake and the mechanical Torsen center differential will transfer torque to the rear wheels.
out on a snowy road, I would prefer a Torsen based-Quattro since it's technically an "active" system (even though the center diff isn't electronically controlled) and will divide torque within it's set TBR, whereas other systems that use an electronically controlled center differential (such as a Haldex-based system) are "re-active" which means it'll only actively transfer torque when wheelspin is sensed.
I've had 328Xi for loaners during snowy days when I lived up in South Dakota, you can catch the system dividing engine torque in unnatural feeling ways in snow and ice situations vs. having a smooth transition.
'14 S5 DSG
'09 E90 M3 DCT (sold)
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