This video, via Road and Track explains 4 types of subaru awd. It says that all of them have a mechanism for sending power toward the less slippy axle (either a viscous differential or a computer controlled clutch differential). They then detect a slipping wheel and apply breaks to send power toward the other wheel.
Fully locked 4WD power is going to go to the wheels that are gripping in proportion to their grip (they all spin at the same speed, and whatever grip there is is the resistance to the engine, so equal and opposite force: all the power is going into the grip, wheels that don’t grip are providing no resistance to the engine).
