The key to the back swing is to turn your center (front part of your torso)
behind the ball without changing your head position and without losing
your balance on the right leg.
Losing your balance to the outside of your right leg and allowing the body,
including your head, to move laterally off the ball is known as a sway.
The opposite is just as bad or worse. That is leaving your weight on the left
side, never turning your center behind the ball, and creating the position
known as a reverse pivot.