Proper Knee Action: Keeping the back heel down (even a bit more) on the downswing will help incorporate fast hip rotation without the back knee moving ‘around/collapsing ’. This will decrease the tendency to come ‘around the top’ (commonly referred to as ‘over the top’) and cut across/block/hook the ball (the result depends on the correction that the hands attempt). It is a fundamental element of a balanced and efficient golf swing.
Proper knee action, along with the sensation and dynamic of dropping the hands on the start of the downswing, will keep you from being too far forward/on our toes (one or both feet) during the downswing and follow through. Improper knee action, especially during the initiation of the downswing, undermines the very foundation of the swing sequence and will result in an inefficient, ineffective and frustrating golf swing.
Proper knee action sets up and works in sync with all of the other elements of a well patterned swing motion (grip, stance, posture, alignment, etc.). This will help you to achieve a consistent and reliable swing, without complex swing thoughts, so that you can get out there and play the game of golf, not the game of golf swing!
So set your foundation and get out there and work on your knee action!