Ideally, your bow arm elbow joint should be rotated so that it is vertical during the shot. This occurs during the Set Up phase of the shot. Rotating the elbow is normally hard to learn, so this drill is a great way to teach yourself how to do it.
For many young or beginner archers, a lack of strength can prevent them achieving this elbow rotation. This is normally due to being “over-bowed” and lacking general physical conditioning. If this applies to you, work to become stronger as a whole. You can see some great exercises that will improve your archery strength in our Recurve Archery Exercises section. Bow training will also help this.