I can try to explain, although this kind of thing is difficult to do without a visual aid.
The gist of it is this: on the end of the sleeve--I believe on the side nearest the body--there is what appears to be an extra layer of microfleece. It's a patch, a square that's roughly the size of your hand (go figure). This square is sewn to the underlying sleeve all the way around, except at the "top" end--that is, the edge of the square that is furthest away from your fingertips.
So the deployment of the mitt is quite simple, elegant even. On this detached edge of the square there's a little webbing pull-tab. You simply pull the tab toward the cuff of the sleeve, and end up inverting the square _over_ the cuff entirely, turning the whole mess inside out, and also exposing a grippy band of leather on what is now the palm of the mitt. I'm very happy with the whole thing. With a half dozen tries the conversion becomes effortless and quick. In dry weather I probably won't be bringing separate gloves anymore.