It is crucial that the right knee remain inside the left knee on the downswing. If the right knee moves outside of the left, this causes the upper body to come over the top and can lead to the club moving too far inside the line of flight, resulting in off-line shots.
To understand the correct action, imagine standing facing a bench that is about knee-high. Take your stance with your knees close to the bench and get into a good backswing position. When you start your downswing, your right knee should move on a parallel line with the bench, never touching the bench. This allows the club to stay on plane and swing down the hitting slot.
But if your right knee gets outside of your left knee, your right knee will hit the bench. This is a sure sign that your swing is not on plane, and compensations will need to be made to square the club face through impact.
To know you have done this move correctly, you will feel your right knee rolling inward. Another image I use to help maintain this proper knee action is envisioning myself hitting a low inside pitch in baseball.
If you do not understand this knee action, your swing always will seem to be in a state of recovery on the downswing. If you practice this right-knee action — regardless of how you take the club back — the club will remain on plane through impact.