FAULT: SLIDING IN BACKSWING
Pivot is a term used to describe how you rotate the upper body against the lower body with your right leg, for right handed golfers. The point of the pivot is to create torque with the upper body against the lower body. A correct pivot also adds depth to your golf swing, meaning that it allows you to swing the club more behind your body. Without a proper pivot you will have no torque and no depth to your golf swing, which translate to no power and a very vertical golf swing. The following drills will allow you to develop a proper pivot.
FIX: CORRECT PIVOT
Using the band to wrap around your knee and then to attach to your belt will give you a great opportunity to create a pivot and power. What you are trying to do is during the setup have the band in a loose position, but at the top of you backswing have the band as tight as possible. The only way to make this happen is to resist you lead knee from sliding and rotate your back hip way from the target line. You should be able to tighten the band with the lower body movement.
Placing a ball under your rear foot will allow you to keep your weight on the inside of your foot, which will also allow you to pivot and not slide back. Working on your pivot could be one of the most important areas you ever work on in your golf swing.