Quattro diff system
 

Sorry if I am being a numpty.

I just jacked a front wheel up, so high that the rear wheel was lifted off of the floor as well.

I turned the front wheel and perhaps obviously the rear wheel turned as well, however it rotated in the opposite direction to the front!

Why is that???? Is it partially because the diff disengages when not ‘powered’....?

Its because of the diffs. If you do the same on end the opposite side will also turn in the opposite direction. Side to side is a standard function of a normal differential. What you're seeing end to end is because the opposite front wheel can't turn, so the centre torsen diff transfers the torque you're putting in to the front wheel to the rear wheel.

If you could jack diagonally-opposite wheels they would both rotate in the same direction :)