The manual has a Viscous Locking Center Differential. If one axle
slips 'enough' the CD will lock into 50/50 power distrubution. The
auto-trannies are capable of variable distribution with the 'duty
solenoid C'. I think the 'corporate' Subaru website, and probably the
Endwrench website will also have explanations. I dunno if the Forester
has a limited slip rear diff or not. If you anticipate regular use on
slippery services, a rear LSD would be very nice to have.
The STI has a different tranny from either the above, BTW.

Carl

Auto trans is 90 FWD/10 RWD until it senses slippage.

Manual trans is 50/50 until slippage.

Manual is much more fun to drive.

The non-STI manual behaves like an open diff until there is 'some'
slippage of an axle. The slippage causes the viscous coupling to lock
the differential. While the diff is locked 50% of the power is at the
front axle and 50% is at the back axle. What happens to various wheels
depends on the type diff on the axle and slippage of individual wheels.

http://www.rallycars.com/Cars/4wd_turbo_cars.html

to research equipment on various models of Subarus, try;
www.cars101.com

also, here's a fun old video about differentials(it gets better after
the intro stuff);
http://www.archive.org/details/Aroundth1937

Carl