A picture is worth a 1000 words, so I will try and scan the illustration in the book - it is basically a double fisherman's with a square knot right in the middle. The stated advantage is that it is easier to untie compared to a double fisherman's, something that has not been a problem for me.

When I am rappelling on a double rope, I really, really want the joining knot to be secure. Even if it should be a bit tough to take apart, that is a trivial problem.

I will try to come up with a picture......