What is Swing Plane?
The swing plane is the imaginary arch that your golf club travels along from address to the top of your swing, and then back to address (impact) and to your finish. A “perfect” swing plane marries your spine angle and shoulder angle. If a line was drawn, the swing plane (sometimes referred to as arch), would be just above the rear shoulder. Anything under that would be considered a flat swing plane and anything above that point would be a steep swing plane.
Your swing plane, when executed properly, should bisect the target after impact. At the lead hip, the club head should be at the target, not to the right or the left of it, but dead on. There are a great deal of factors that can influence the swing plane.
Ben Hogan was so good because his plane was pretty much perfect. He worked very hard on making sure the club returned to its starting position (see picture below).