The best way to save energy in swimming freestyle is to rotate from side to side.
We rotate for a few reasons. It is important because it reduces the amount of drag our body creates in the water. Does it waste energy going side to side? It does and it doesn’t. Rotation does use energy (only a small amount) however we are using the momentum of our stroke to get from one side to the other. Also, by rotating it allows you to anchor your hand in the water and to move past that anchor much faster and easier than you would by swimming flat. Thus faster swimming!
How far should I rotate? Depending on your ability, most elite swimmers will rotate to about 45 degrees. This doesn’t mean you can’t! It may take some practice before you get this angle comfortably, but it is worth the effort once you do. Long distance swimmers generally rotate more than a sprinter would. If you are racing a triathlon or more than 400 meters in the water, work hard to rotate as much as possible for a longer stroke. During sprinting, it is best to limit your rotation slightly in exchange for a higher stroke rate.
What is the best way to work on my rotation outside of the pool? The best way is to increase your core strength (abdominal strength). This is good news! For guys it means you get to work on that six pack abs, and girls get will see a flatter stomach doing these exercises. The best methods for increasing core strength are prone hold and pilates. Prone hold is simple and can be done before bed each night, and pilates is an excellent way to improve your swimming muscles.
An important point in body rotation: Make sure your hips and shoulders stay connected. This requires a strong core as we mentioned earlier. Your shoulders should rotate with your hips and should move at the same time. It can work best by concentrating on moving your hips first and your shoulders second.