I usually figure things out while doing something else- my homework suddenly clicks as I walk to the bathroom, or I realize on my way home from a date that I should have kissed her. This time it was swimming technique during quantum mechanics class. As I was listening to the lecture, I figured out the reason for the S-curve.
Earlier that day my swimming teacher had gone over the S-curve, which is the way you pull your hand through the water in the freestyle stroke. It felt smooth as I practiced and it definitely made me swim faster. But it wasn't until later that I figured out why it works so well. First I'll show you S-curve, then I'll explain my realization.
You reach forward and as you put your hand into the water, you (1) rotate your wrist so your hand points slightly outward and pull out and down (down being toward your feet, because your body is parallel to the bottom of the pool.) As your hand gets lower you (2) rotate your wrist again the other way and pull toward you and down. And when your hand gets to about your waist you (3) rotate the wrist again and push straight down. Here is a diagram:
Most beginners pull straight down so your hand moves in a half-circle with the elbow mostly straight, however the S-curve has been found to be much more effective. But why?
It has to do with the muscles in the upper body. When the arm is fully extended, the muscles that would engage the most to pull your arm down are your lats. But your lats are the strongest when they pull not just down, but out. So you play to your strengths and pull down and out. Then when the arms get further down and the elbow is flexed, the strongest muscles are your pecs and biceps. And these muscles do their best when they are pulling inward; therefore, you pull your arms down and in. And finally when the hand is low at your waist, the muscles you use to extend your arm are your triceps. And these work best pushing straight down, so you do just that and push your hand toward your toes.
Do you see the pattern? In each case you are using each muscle group in the ways that they are the strongest, thus giving you the most power possible at every stage. I definitely noticed that my arms were sore in different places because I had been recruiting more muscle groups. So after only one swim and a little time to think it over, I am definitely a believer.