I happen to do a lot of competitive swimming. I practice 6 days a week, 2-4 hours a day. As one might expect, swimming butterfly might be pretty important. I learned a lot about how I could improve my stroke from this video, and make it so I do not die every time we have a "Fly" day.
The first thing that I learned applies to the kick of the Butterfly stroke.
If you notice, the swimmer uses a relatively straight leg while kicking and keeps his legs low in the water. When I swim, I do the exact opposite, bent legs and and high in the water. I do this in order to keep my body flat which reduces the amount of energy used. However, for a race, this is not appropriate, as my style is slightly slower because not as much power is used. Straight legs allow you to use your kick in order to support your body and throw yourself out of the water.
The next thing that I learned applies to the pull of the Butterfly stroke.
If you look carefully, you notice that the swimmer uses a bent wrist, and pushes down with his forearm in order to move in the water. This is efficient because not only does this push the water away and off your body, it propels you through the water. I pull inward and with a straight wrist until I meet the middle of my stomach. This is not efficient because my face slightly bends upwards which makes me sink, and lets the water push against my face which can take .4 to 1 second off my time. This is a lot in swimming.
The final thing I learned applies to the recovery and breathing method in butterfly
The first thing applies to the direction the swimmers hands are facing when the come out of the water. The face downwards as so to begin the pull phase as soon as they hit the water. My hands are straight, and swing out ward, which uses more energy and makes it harder to swim. The second thing applies to the head position the swimmer uses when he is breathing. If you look closely, you can see that his head does not move or turn as it comes out of the water. My head perks up which slows down my speed as the water pushes against my face as I reenter the water.