With older X5's it was a 60/40 permanent split.

Your X5 has xDrive (04 and onwards), which means that there is a clutch that can variably transfer power from the front to rear wheels in any discretion it chooses, instantly and without any intervention from the driver.

So for example, you hit a puddle (with your front wheels). The car knows this, and so all the power from the engine goes to the rear wheels. Once your front wheels clear the puddle, power starts coming back to the front wheels, until your rear wheels are completely in the puddle, and then all the engine power goes to your front wheels. Once both wheels clear the puddle power is variably restored.

Now the interesting thing about xDrive is that it varies power at all times. So you can be taking a curve too fast and power will be moving ever so slightly. Or you could be going over different road surfaces (say like different concrete on a freeway) and xDrive is adjusting to that. Obviously the changes in drier conditions are less noticable and less extreme than on wet conditions, but my point is, is that xDrive is always working...

My explanation might not totally be clear, but BMW has pretty good visuals on its website explaining what happens with xDrive... If your interested I can dig up the link...