Although AWD's purpose is to prevent cars from losing traction, you can really use it to your advantage to keep the wheels spinning. If you recall the "traction circle" that you learned in performance-driving kindergarden, you'll remember that the East and West directions represent steering right and left and the North and South directions represent acceleration and braking (respectively). Now, to make a car lose traction, the load on the tires must exceed their ability in any one of those directions or as a combination of two (or more, but that's a bit more advanced). Let's consider 2 cars with the same specifications, car A having RWD and car B having AWD. When accelerating from a stand-still, car A will be sending 100% of the power to the rear wheels only. Granted that car A has the ability to load the tires beyond their capacity, it will break the traction circle and spin the rear wheels. If car B were to send 100% of its power to the driveline, it would momentarily go 100% to the front wheels (in the TSi's case) and break the traction circle causing wheel slipage. The slip sensor will detect this and tell the transmission to take up to 50% from the front and send it to the rear. When it does this, the load on the front tires will move to inside the traction circle and the rear wheels' traction circle will look exactly like the front's (ignoring weight and transfer). Simple, right?

Now imagine when a car is going around a turn and power is being applied while there is a constant latteral load on the traction circles of all tires on both cars, during a left turn for example (meaning that the work done on the tires is along the East-West line on the left side). While cornering at half of the capacity of the tires, car A can apply enough power to exceed the traction circle of the rear wheels (the work done by the tires is outside of the traction circle on the upper left side). Meanwhile, the front tires will stay at their half-capacity of lateral load without ever knowing what is happening to the rears. From my unserstanding, once a driver counter steers, the direction of lateral load on the front tires is reversed (to the right in a left-turn drift) and they are now working to prevent the car from going off of the track to the left - there's a split second in a feint where the rear is sliding and the fronts are pointed straight just after the body roll has passed neutral. In our left turn, if the driver does a power-over and applies just enough power for the rears to break traction, he can keep the work asked of the tires just beyond the traction circle and maintain control of it, but if he applies too much power the tires will be overwhelmed and the driver will lose control -- this is why higher-powered drift cars need to have the power modulated in order to keep control and keep from spinning while lower power cars can be floored and stay in control.

All-Wheel Drifting techniques require knowledge of vehicle dynamics, your drivetrain, your AWD system, your traction circle (the best drivers are subliminally imagining all 4 traction circles of all 4 wheels all at the same time all the time). I find that the majority of the time all wheel drifting is spent trying to find ways to "trick" the computer into giving more power to the rear wheels than they can handle. Rocking the steering wheel between neutral and counter-steered one direction is a pretty good way to keep constant load on the outside tires and to find the best steering angle.