If you ask swimmers what their hardest stroke is, most will respond with, butterfly. Butterfly is usually the most difficult due to its pull and kick. Though many of the swimmers don't notice it, the kick is atually the strongest kick. This is why swimmers do this underwater once they dive in. This kick is called "Dolphin Kick". The pull sounds simple but is definatly harder then it looks. Below are some diagrams showing how to do so.