Nowadays, particularly since Hank Haney came to prominence as Tiger Woods swing coach in 2004, the swing plane as a fundamental in the golf swing has received significant attention.
However, there is widespread disagreement about what it really means. One analyst or teacher may be referring to the plane of the shoulders in relation to the golf ball, while another refers to the swing plane of the golf club.
The idea of the swing plane dates back to Ben Hogans book Five Fundamentals. In this book is a famous drawing of Hogan with a pane of glass drawn through his shoulders. Hogan indicated that you needed to be sure to swing the golf club under this swing plane.
In the case of Hank Haney, the swing plane refers to the plane of the golf club as it moves through the golf swing. Ideally, Hank says, the club will stay on a swing plane angle that is parallel to the original angle of the club established at address.
This will make it easier to return the club to this same angle at impact. By returning the club to this angle at impact, the golfer has a much higher chance of hitting the ball with a square clubface, which allows for more solid and consistent ball striking.
In the video below, I discuss the swing plane in more detail.