The good backswing puts you in a position at the top to deliver the clubhead to the ball square and on line to the target. A key determinant of that position is the stability of your right leg and knee during the backswing.
One way to say it is that the right leg is a post around which the backswing rotates. The right leg does not change its position from address to the top, and the knee retains its flex. This is something seldom talked about in instruction books, but if you look at swing sequence photographs of tour pros printed in golf magazines, you will see this consistently.
The benefits of this move are:
you create torque that springs the club down to the ball rather than starting down from a dead stop (= free power);
it takes the slack out of your swing, eliminating room for your swing to get out of line;
your weight stays centered so it can transfer naturally to the left side on the downswing; and
since your weight stays centered, your balance improves.
Odds are that right now you’re doing the opposite – letting the right leg sway to the right and letting the knee straighten out as you take the club back. You do that because it feels comfortable. Remember, the golf swing isn’t based on what’s comfortable. It’s based on what’s right.
Keeping your right leg steady is a professional move. It takes practice to build it into your swing. Learn it gradually by doing this exercise.
Take your address position, without a club, feet apart, knees bent. Stand straight up but without taking the bend out of your knees. Turn to your right (to the left, for lefties) without letting your right leg sway to the right, and keeping your knees flexed. It will help to think of your right hip turning back. Your left foot can come off the ground a bit if it has to. This exercise should feel fairly comfortable after a few tries. Keep doing it until you can keep your right leg and knee in position without thinking about them.
Now bend from the hips into your address position and turn the same way. Just let your upper body follow. In your first attempts it will feel like this move restricts your turning. That’s all right because it’s supposed to feel like that. You’re taking the slack out of your turn. What is being restricted is the room for your club to wander off course in its journey away from the ball and back to it.
When you have done this version of the exercise enough times that it feels natural, pick up a club and do the exercise. Just make a backswing, over and over, keeping your right leg in place and your knee flex unchanged. Even with steady practice, plan on several months for this move to start taking hold.
The work will be worth it, though. In time you will begin to feel the power that you are storing up, you will feel your are in better balance, and you will feel like the clubhead has nowhere else to go but smack back to the ball. It’s a wonderful feeling, really.
When you finally hit some balls with this move built in, don’t be startled at how powerfully they fly down the range.