When the motion is done on a horizontal plane, you can see the full scope of the error. It's not that it can't be done. But, there's no way to justify the exaggerations that I've seen practiced. It's troubling that the camera angles of Mr. Hardy, seen on YouTube, are at 45 degrees from the plane angle.
I would say that the failure to bend the right arm with the pulling of the right elbow behind the body cause the club to move under plane.
I have nothing against Mr. Hardy. I've heard nothing but praise for him. But, I can hate the motion, if it breaks laws.
I really like these two pictures from Daryl, and I think they do a great job of supporting your position. The lawn mower move will not comply with our geometry or the proper use of extensor action IMO.
Kevin
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I could be wrong. I have been before, and will be again.