The extreme one-handed backhand grip is where the heel pad and index knuckle of your hitting hand rest on the eighth bevel of the tennis racket handle (one bevel counter-clockwise from the very “top” of the handle).
At 0:30 in the video above you can see me holding the racket with an extreme one-handed backhand grip, and both my heel pad and index knuckle are resting on that eighth bevel of the tennis racket handle. A simple way to find the extreme one-handed backhand grip is to get yourself into the semi-western forehand grip, and then turn your arm and the tennis racket over to your backhand side. For a visual explanation of this, see 0:45 in the video above.
You may ask yourself, Why don’t I just play with a semi-western forehand and an extreme one handed backhand grip so that I never have to change grips for groundstrokes?” In theory, I suppose you could do this, but I do not know of any player at the pro level who uses both a semi-western forehand and an extreme one-handed backhand grip. Firstly, it forces you to hit a semi-western forehand, and second, the extreme one-handed backhand grip is a tough grip to master. A player like Justine Henin had an amazing one-hander using this grip, but most players use something a little more conservative.