Involving your legs in your downswing is a key component in utilizing the power your swing creates. If done properly, you can add an extra 10-20 yards to your tee shots. To do this, your left knee needs to kiss the right one as the club comes into impact. Among my students I see a lot of stationary legs; this contributes to their lack of distance. Your right knee should be driving towards to the target as you approach impact; this move helps promote a proper weight transfer.
As shown in the picture, imagine yourself holding a balloon between your knees. As you swing back, try to keep the balloon stationary. This will insure you’re coiling properly and creating tension between your upper and lower body. Furthermore it will also insure you’re rotating your body, rather than sliding horizantally (which often leads to a push). When you’re approaching impact, feel as though your right knee is popping the balloon on the left. This move will help you clear your hips properly while transferring your weight, leading to greater clubhead speed and distance.
Give it a try!