FAULT: SLIDING KNEES
Fixing the issue of sliding knees or weak knees is really not that hard, the drill below should help get rid of this fairly quickly. The knees help create a stable base for the hips to rotate around, so if the knees slide or get weak this prevents the hips from making a proper pivot.
FIX: STABLE LEAD KNEE AND PROPER PIVOT
Place the band around you knees so that there is a light pressure. Then take some practice backswings while trying not to let the band fall to your ankles. The bands should only touch your ankles when you are holding your finish. During the golf swing the knees will come together, but in the setup and the backswing the knees should keep their distance.