[O.B.Left]

For maximum compression, the clubface must close, relative to the direction that it is traveling, during the impact interval. This is a horizontal hinge motion of the clubface, and is required to keep the original contact point between clubface and ball intact throughout the interval. Also, the clubface must not "lay back" during the interval, which would add loft, permitting compression to "leak" away.

Now if I were to explain D Plane to a 6th grader, I think I'd go with this.......When the clubface hits the ball, the clubface is moving on a line, and the clubface is also pointing on a line. If you take your pencil and connect those two lines with another line, that new line is called "The D Plane". Not so scary, eh?

In regard to the last bit .......Im trying to picture that. Do you mean one line is arching and the other is straight ish and you connect them at the far ends in a three dimensional drawing? Or are you talking 2D, plan view? Not being a jerk or anything, I really dont know what you mean but it sounds so simple.

In regard to Horizontal its not just any old amount of face "closing relative to the direction it is traveling". The face via the left hand maintains a vertical relationship to the Horizontal Basic Plane , the ground. It can over done or underdone , the identification of the three Basic Planes bringing precision to the execution.