In an efficient backswing, you rotate your upper body and your hips and coil into a braced trail leg. This creates a huge mount of rotational energy and it's what allows you to use the ground for power. Keeping the trail leg braced and the weight on the inside half of your trail foot also keeps your turn centered, making it much easier to return the club consistently to the golf ball.
But when you sway, when you let the trail hip move laterally outside of your trail foot, you lose that powerful coil. You also make it harder to maintain balance, you make it necessary to sway laterally during the downswing to get back to the ball and you also risk injuring your hip and lower back.
To correct this common swing fault, all you need is a club and a chair, as Titleist Staff Instructor Trillium Rose demonstrates in this video.
Anti-Sway Drill
1. Stand to the side of a kitchen or dining room chair. Take note of how far your trail hip is from the edge of the chair back.
2. Swing slowly to the top of your back swing and hold that position. Check how far from the chair back your hip is now. Your trail hip should rotate back and the hip should be no closer to the chair than it was at address.
3. Check your weight distribution. The majority of weight should be in your trail foot, favoring the heel, and the weight should be centered or even favoring the inside half of your foot. Your weight should not roll to the outside of your trail foot.
4. As you make practice backswings, imagine your right hip pocket (for a right-handed golfer) moving straight back behind you as you rotate. Feel some pressure in your trail leg, foot and glute as you coil into your trail side.
