How does 4x4 really work

Last night got a bit stuck in a big snow bank, Selectrac I6 4x4, i was on
full time

The rear right (passenger) wheels was spinning and it feel it was the only
wheel really trying to get out, i know the 4x4 is working (Light on and
feeling while doing u-turn ) but why i did not see the front wheel spinning
while the gc was on Reverse , I was outside with the truck engage looking at
the truck spinning. !!!!!

I tried part time and no much luck. Got out of there with time and patience
REVERSE,DRIVE,REV... Is AWD subaru vehicules working the same ?

thanks

Pat