According to Hogan the arms move at an angle under or just brushing the plane. Of course if the lead arm was brushing the ball to shoulder plane at impact the clubhead would be well off of the ground.
This is due to the fact that the clubshaft and lead arm do not form a straight line when viewed from behind.
So, I would say that Hogan returned his lead arm on a path (plane?) that got it to impact position,
leading me to wonder exactly where the vertical drop is.
Is it thru the entire downswing?
Is it only at the top of the downswing?
Is it somewhere else?