How did his ballfight change with the two different swings. Did his misses get better and/or did he gain better control of the d plane? And why?
The swing change gave me a ballflight that is considerably higher, and I can now intentionally move the ball from right to left. For example, the "after" swing produced a high, straightish shot that fell ever so gently to the left.
I went to the range yesterday and would say that the majority of the shots I hit were high fades. Some started on line and others started a smidge to the right. I have some work to do as far as becoming comfortable with the new move and being able to reliably draw the ball, but I'm hitting the ball WAY better than I was before. Prior to my lesson my predominant miss was a thin, weak push cut that invariably came up short. I can still hit that shot, but I find my new miss to be a high fade that is struck solidly, a little right, but more or less pin high. I'll take it.
We didn't talk about D-plane specifically, however, Brian did get me to stop aiming so far left, which I assume predisposed me to hitting a cut. I think once I can execute the new swing a little less deliberately I'll be able to start thinking about the D-plane and how I can use it to make shots.
I can't help you with the "Why" part of your question, I think Brian will have to chime in on that.