[Mike O]

We're talking about "underplane" when the clubshaft doesn't point at the plane line (instead it points outside of it). A shift to a flatter plane angle with the shaft still on-plane isn't that big of a problem, but being flat and off-plane is a big problem. You do come into impact feeling too much inside-out and are usually left hopelessly unable to control the clubface. What feels like you "driving to the inside aft quadrant of the ball" is in reality you dropping the clubshaft off-plane and driving to outside the plane line.

Thanks Mike; your thoughts would be much appreciated.

Thanks,
I've got the image now. Don't really like the term "under plane"- although if well defined as mentioned then no problem.

If I were to translate the same thing to a different description I would say that it's:

The flattening of the plane on the downswing while changing the plane line from square on the backswing to closed on the downswing. If you've got your "straight shot" grip to begin with and then change the plane line coming down to way right- essentially no way to square the blade without throwaway.

Although, this would be something that you would really want to check on video.

Usually, things in the golf movement come in pairs. If you've got one problem then you've got another to offset it. So if you've been coming over the top- you might also be blocking the shot to prevent a big pull. Now, for that person - coming down on plane without correcting the "ingrained block" would create an "under plane" feel and an "under plane" ball flight. So instead of trying to not come down "under plane" they would want to make sure while they were coming down that they allowed better rotation of the face. So case by case. Not saying in this last paragraph that you guys were not saying that this possibility existed, or that you were not aware of it, I'm just throwing it out there since it hadn't been explicited posted.