Quick answer: Face-on: the trail hip slides laterally outside the trail foot during the backswing instead of rotating; the head travels several centimeters to the right.
What it does to the ball
Unpredictable low point: alternating fat and thin shots, because getting back to the ball demands a reverse slide that is very hard to time.
Root causes (by frequency)
Limited hip mobility that swaps rotation for a slide (TPI screen)
Trying to load the trail side in an exaggerated way
How to fix it
Turn the trail hip back behind you as if coiling, with pressure on the inside of the trail foot and never spilling outside it
Drills that fix it
The hip post · 15 min · 15 reps
Swap the sway for pure rotation Stick an alignment rod in the ground (or use your bag) brushing the outside of your trail hip. Make backswings turning without pushing the rod: the hip rotates, it doesn't slide. Then hit balls at 70% without touching the post.
Pressure on the instep · 10 min · 15 reps
Load the trail side without spilling outside it Place a golf ball under the outside edge of your trail foot. Make swings feeling the pressure stay on the inside of the foot throughout the backswing. Then hit balls without the ball under the foot, chasing the same feeling.
Frequently asked questions
What is "Sway (lateral slide in the backswing)" in the golf swing?
Face-on: the trail hip slides laterally outside the trail foot during the backswing instead of rotating; the head travels several centimeters to the right.
What does sway (lateral slide in the backswing) do to the ball?
Unpredictable low point: alternating fat and thin shots, because getting back to the ball demands a reverse slide that is very hard to time.
How do you fix sway (lateral slide in the backswing)?
Turn the trail hip back behind you as if coiling, with pressure on the inside of the trail foot and never spilling outside it
Sources
Titleist Performance Institute — Body-Swing Connection screening