When you learn to swim Butterfly style, first you need to train your flexibility and fluidity. This will be useful not only in Butterfly, but in any other styles of swimming. Remember, that flexibility and fluidity are the key to the easy and correct swimming. To achieve that, we need the "dolphin" movement as an example.
To start with, we try to copy the movements of a dolphin which has to go under a series of obstacles on the surface. To make it easier, it's better to use fins in order to feel the strength of the "dolphin tail" kick. In this exercise we mostly use hips and legs.