O.B, HK is referring to putting when he says "Steeper than normal Elbow Plane".
Place the Grip in the cup of the Left Hand and adjust the right hand to correspond, then reach out with the arms to move the Right Elbow to the Steeper Plane. Then the Right Elbow, Right Forearm and Clubshaft are on the same Plane throughout the stroke. It's a "Straight Line Path" Elbow Plane. I'm sure that someone can make longer strokes and maybe use a Pivot Stoke too.