A common mistake for many golfers is to straighten or lock the back leg on the backswing. By locking that knee, you lose the usage of your lower body and have to rely on only your upper body for your power. Besides losing distance, players often develop an “over the top” swing which leads to pulls or slices. A easy drill you can do at home is simply to take a backswing, stop at the top, and check to see that your knee is still flexed like it was at address. This is a crucial piece to staying athletic throughout your golf swing. A simple phrase I like to use when instructing is “turn onto a flexed back knee”. Work on this position and your power will return.