Saw something interesting from Martin Hall on Golf Channel recently discussing variations in swing planes, specifically one- and two-plane swings. This is a concept championed by Jim Hardy.
Martin said that a one-plane swing has the shoulders pointing more downward with the angle of arms matching the shoulder angle in the backswing, while a two-plane swing has the shoulders turning on a flatter plane with the arms moving more vertically during the swing (what I do currently).
He went on to say that the two-plane swing relies more on timing with the arms on the downswing, while the one-plane's swing relies less on timing because of the pivot used to square the face.
Chuck Quinton's Rotary Swing would be an example of a one-plane swing.