From Dana Dahlquist.
The plane line is runs parallel and right next to the target line. As Dana is showing, the 'plow stick' is pointing at the plane line in the backswing. That's a 'straight plane line.'
Of course, you can use the Smart Stick to help as well. Whatever you do, 'FEEL YOUR WAY AROUND IT.' What does it feel like to other parts of your body? What is your forearm doing? What's your elbow doing? What's your shoulders doing?