The plane concept can be confusing, and many golfers are unclear about what it is and its affect on your golf swing. To keep it as simple as possible, the swing plane is the path the shaft of your club travels as you swing. There are several reasons for swinging off plane, but the most common reason is using the wrong spine angle at address.
The spine angle creates a sort of natural axis around which your shoulders pivot. The spine angle you set when you address the ball decides the plane of your swing. If you lean too far over at address, your shoulders will tilt during your swing, causing your left arm to come off your chest. This will create a plethora of problems–pulls, slices, deep divots, fat shots, etc.
If you lean too far back, your shoulders will flatten, pushing your left arm too tightly against your chest. Thin shots, loss of power, and hitting behind the ball are all results of hitting on this shallow plane.
In a good backswing, your left arm is on a higher plane than your shoulders. This allows you a direct line to the ball on the downswing. If your left arm swing isn’t correct, you’ll be on the wrong plane and your swing will be off.
To check the accuracy of your plane, take a club and assume your normal setup posture. Take a swing to the top and hold that position for a second. Then loosen your grip and let the shaft drop. If the shaft hits the top of your right shoulder, your swing is on the proper plane. If it hits you on the head or neck, your plane is too steep. Your plane is too shallow if it falls behind you without hitting your body at all.
To correct this, try a simple move at the top of your backswing. When you begin your downswing, shift your weight onto your left foot while bringing your right elbow down to your body–just make sure you’re keeping your wrist angle the same since that’s where your power and distance come from. As the weight shifts, the club will automatically fall into the correct swing plane and brings the clubhead squarely to the ball.
This creates a shot using two separate planes: one for the backswing and one for the downswing. This is critical for a shot with a square clubhead and good power.