Rich is correct in that only the rears can disengage, the fronts will always have drive. As mentioned earlier, the S3 is pimarily FWD up until the point that the wheels slip, then the Haldex engages to transfer power/drive to the rear wheels, until all wheels are going at the same speed. The question is drive ratio split between front and rear, which in this case is 50:50 F/R under most extreme conditions.
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1. There are situations where near 100% torque transfer to the rear axle
occur. An example is if the front wheels are on ice and the rear wheels are
on tarmac. In that case the front wheels have (almost) no grip. In that
case, the Haldex coupling will transfer all torque to the rear axle and
prevent front wheel spin. On uniform surfaces however, the coupling can not
transfer all torque to the rear axle. See below.
[/ QUOTE ]
What the quote above means is that with the drive being 50:50, then theoretically, with the Haldex engaged and all four wheels travelling at the same speed and the fronts can't get any grip(front wheels on ice), then all the suppose drive/power is left to the rears if it were in contact with tarmac. Make sense?