Its a common misunderstanding that the haldex only engages the rear wheels when the fronts start to slip. This was the case with the first gen haldex, but the newer versions are more complex:-
From a standing start , if the front wheel starts to slip (by something like 6 degrees) the rear wheels will engage.
if the car is already rolling, then the computer that controls the haldex sends a varying amount of power to the rear depending on various inputs, such as throttle position, speed of throttle press, yaw/pitch/roll angles etc. So its constantly varying the amount sent backwards between 0% and 99%, although in practice you would rarely get 99% at the back.
In practice, It works brilliantly, the car never feels like a front wheel drive car. From a standing start in the wet, if you dump the clutch (and drive like a ****t), all four wheels will spin up and its impossible to tell that the fronts span up slightly before the rears.