Two of the biggest myths in golf instruction are “keep our head down” and “keep your left arm straight.” First, people must understand that you don’t miss shots because your head comes up. In fact, it’s because of poor body movements. Second, the thought of keeping your left arm straight will create tension and impede speed in your swinging motion. Also, if you try to keep your head down, you’ll wind up not properly releasing your weight to your left side during your follow-through motion.
So let both arms relax (even your left), allow them to swing with freedom, and allow your entire body (including your head) release to a full level finish. You’ll hit better shots with great ease.