From what I understand in alot of the systems, regular driving yeilds 90% front 10% rear power distribution, however it varies depending on load and grip. For example, if you accelerate uphill, more of the vehicle's weight is transfered to the rear and the PCM distributes more power to the rear wheels. If you are braking or descending a hill (more weight on the front wheels), more power is applied to the front. Also, if say, the front left wheels slips, it distributes all the power to the other three wheels, or whatever of the three has more grip.
That's without looking anything up so I could be corrected on some of it. Subaru definately has the awd system down pat though!