Throughout the years, the swing plane has always been one of the most difficult things to understand. Hopefully, I can simplify this difficult subject so you can achieve the proper swing plane in your golf swing.
I like to use the image of a weight swinging on a piece of string to illustrate the swing plane. What would happen if you picked up a weight on a piece of string and started to twirl it? Assuming your hand was on a 45 degree angle in front of you and not tilted left or right, the weight would make a circular arc around your hand causing the weight to swing down the line. If the weight was allowed to swing freely, it would do the same thing on one side as it does on the other. That means it would be swinging on a perfect plane around your hand.
Now that you have an understanding of what the swing plane is, you have to also understand that there are 3 different planes you could have. To imagine the different planes, think again about the weight swinging on the string. We already discussed the down the line plane (hand not tilted) but what would happen if you tilted your hand to the left? If you tilted your hand to the left the weight would swing to...