It looks like this:
I haven't had a chance to tinker with it yet.
It should be pretty sweet.
It uses these plastic templates or guides to like "track" the router to make the mortise and the tenon.
Just draw a crosshairs on your stock and line up the jig's sight:
There is a tapered pin that rides in those "tracks" of the plastic templates.
So to tune the fit of the mortise and tenon joint, you can screw that pin in or out.