Many golfers have issues with sway in the backswing but few understand the consequences and what the swing tendency actually leads to with respect to ball flight. When we say sway we are referring to the pelvis moving laterally away from the target in the backswing. Here is an example of a chain reaction that can happen when the pelvis sways in the backswing:
When the pelvis sways off the ball there is a strong tendency for the upper body to lean towards the target which is a swing flaw referred to as ‘Reverse Spine’. From the reverse spine position the golfer does not have leverage to support the golf club in front of the torso which means the hands will tend to work deep behind the player. From this type of position at the top of the backswing many players tend to come over the top in an effort to get the club back in front of the torso. Once this has happened the golfer will instinctively react to shallow the path of the club which leads to early extension. The ball flight tendencies associated with early extension are many, but blocks, hooks, and fat shots are just a few. So you can see that sway is really more than just sway, it can lead to a chain reaction in the swing which makes controlling the golf ball at top speeds very difficult.
The first step to eliminating sway in the golf swing is to understand why it exists in the first place. In most cases there are physically limitations such as a lack of lateral stability in the hips that contribute to the tendency for sway. Getting a proper physical assessment is a great start to progressing towards a more efficient golf swing.