I understand it now after inspecting the slipper. See picture below.
The pulley that rear belt connects to will slip independently of the shaft. So when I turn both rear wheels while holding the front wheels I was expecting to see the end of the slipper shift (where the nut is screwed on) rotate. It doesn't. If it does then either 1) the belt is skipping or the front diff is slipping.
Looking at the slipper where the front belt sits, the whole shaft will slip when holding the rear wheels while turning both front wheels - this is the correct behaviour when not running the clicker as the front belt pulley is seated on the shaft and rotates with the shaft.
So setting this thing up is a pain as you can't see the slipper when adjusting it.
Hopefully this will help others.
