Where the plane intersects the ground creates a line. A straight plane line.
Regardless of how flat or upright of a plane you are swinging on that line on the ground does not change.
If you had a laser light coming out of both ends of your shaft they would always trace this straight line on the ground. Except when the shaft is parallel to the ground.
That should help, notice how the either the butt end of the shaft or the sweetspot of the club is always pointing at the line UNLESS it is parallel to it.
So if I stick two flashlights at the end of a pvc pipe and lay a couple clubs on ground the light should look at the clubs except for when the club is parrallel to them right?
I learned how to move the right shoulder down plane by putting a driver across my shoulders with the head out the right shoulder. Then you simply turn back and forward and try to hit the ball with the head of your driver (which you can't since it is only sticking out about two feet). That will give the feel and movement you want the right shoulder to do.
I saw this done on the Golf Channel by Martin Hall and it works great.
There are more detailed instructions for this exercise somewhere on the forum but you should get the idea.
It really helps if you move your lower body first (slight lateral movement) which clears the way for you to get the shoulder down plane.
from the top down it should be, shift hips lateral just a bit, right shoulder drops toward the while the head remains back? when should you start rotating your hips? through impact?