Um, I don't think this is traditional "underplane" as that usually means dramatically in-to-out path, ie, the clubhead is outside the target line after impact, as there usually is some type of impact. He is saying that the club is passing between his feet and the ball, so this is quite a bit more sinister than too much from the inside.
His whole plane is displaced inward and not intersecting with the ball at all. The club hits the ground approx where the shaft is pointing on the downswing. So, the down-arching of the wrist/forearm is steepening it too much--the shaft is no longer pointing at the target line....enough so that it is completely missing the ball.
Curtis, try rotating your forearms to close the face instead of down-arching. Clearly this is an experiment that has gone hideously wrong.
PS: And in response to your thread title, no, I've never missed the ball from the inside, but I have gotten steep and toe'd the piss out of it....which would be a kind of Egor compared to the Frankenstein you've created.
