What is a SLIP DIFF on an mx 5

[trace1967]

I have seen two 'S' Ltds for sale on the classifieds and they look gorgeous and better than mine. But what is a slip diff, as they seem to have this, I am not sure mine has it. My mini-steering wheel horn is not working and I took the steering wheel off to have a look and the contact was clipped on but waggly, but it still does not work. I am thinking of changing the wheel back to a new normal size one, but which one should it have as the original???