3. Body Rotation
Body rotation is somewhat related to reaching forward, in that by pivoting your body with each stroke, you facilitate your shoulders extending forward at the end of each stroke. When your right arm is fully extended in front of you (and your left is about to exit the water behind you for its recovery), your body should be pivoted right.
This means the entire right side of your body should be submerged and facing the bottom of the pool, while the entire left side of your body should be breaching toward the ceiling/sky. With the next stroke, your body pivots to the left, altering your position about 120 degrees. Picture a rotisserie chicken being pivoted on the axis of the spit, and that is how your body should rotate with each stroke.
4. "Hourglass" Pull
When you are pulling your body through the water with your arms, you want to maximize the amount of water pulled. Since the shortest distance between two points is a straight line, the last thing you want to do when swimming is pull your arm through the water in a straight line. Instead, practice an S shape (a longer way of completing each stroke), so that if you were to pull both arms together simultaneously, the resulting path would resemble an hourglass silhouette (this simultaneous silhouette should also diagram your butterfly underwater pull).
At the beginning of the stroke, the hand extends out, away from your body. Keep your elbows high. As you catch the water, curve your hand back inward toward your belly button, then out again by your hip as your hand exits the water.
5. Finish the Stroke
Even some of the world's best swimmers end up shortening their strokes when they get tired, pulling their hands out of the water prematurely at their waist area rather than by their upper thigh. As your arms complete their underwater hourglass pull, they should fully extend behind you, by your sides, so that your thumbs graze the side of your thighs below your suit-line.
Many swimmers begin bending their elbows toward the end of their stroke and pull their arms out of the water before allowing them to finish their path. By shortening their stroke, these swimmers lose efficiency while actually expending more energy because they are taking more strokes per lap (essentially spinning their wheels).
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A former swimmer at Stanford University, Alex Kostich has stayed strong in the sport at the elite level even while maintaining a day job. The three-time Pan-American Games gold medalist still competes in—and wins—numerous open-water races around the world each year, as well as competing in the occasional triathlon and running race.