Swing plane describes the path and the angle of the club shaft as it moves around your body when you swing.
If you swing on one plane you will tend to have a more bent over spine angle at set up and during your swing. You will stand further away from the ball than you would using a two plane method. The stance width will be wider than that of a two plane swing, to provide a good base for the aggressive torso rotation of this method. The left arm will look more snug to the body on a one plane takeaway and the right elbow will move up and behind the body more than the movement seen on a two plane swing. The shaft of the club will be parallel to the target line but more on the inside than the position displayed by a two plane golfer.
Golfers swinging on a one plane swing rotate their shoulders more steeply than those using a two plane method and their arms swing up on to the same plane as their shoulders. One plane golfers have a more rotary movement and less lateral weight shift to the right (right handed golfers) than two planers, so the weight will stay more centred and the backswing will feel lower and deeper.
As the club is being swung more around the body with this method, the left arm does not disconnect from the chest and as a result, the left arm will look parallel to the shoulders at the top of the swing and will not look as high as the position displayed by a two plane golfer.
On the downswing, a one plane golfer will have their right forearm and the club on plane before the strike and their body will continue to rotate hard through the strike and into the finish. The club will move back to the inside early on the follow through and the shoulders will unwind to finish aiming left of the target – for a one plane golfer the more the shoulders rotate left, the better!
Try this drill to determine whether you are suited more to a one plane or two plane golf swing. First, stand further away from the ball, bend over and hold your driver like a hockey stick. Swing the club around you on a flat plane and hit some golf balls. Once you have done this, stand more upright and hold your driver with your usual grip. Swing back and then push your hips towards the ball before you swing your arms through.
If you hit better shots with the hockey style swing, you are probably a a one planer. If you hit better shots from the more upright position, you are likely to be a two planer.