A butterfly kick is an undulation of the torso and
the legs in a vertical plane it starts with a drop in the hips, then the
knees and finally a flick downwards with the feet. If you want to practice
it try the following underwater (you can’t do fly kick effectively at the
surface).
Imagine your legs are bound together, or even do bind them by
cutting the top of an old swimming hat and using it like a large rubber
band, push off the wall well below the surface with your arms stretched out
in front of you. You’re not allowed to pull your arms back, you can’t move
your legs up and down separately. How are you going to propel yourself
forwards? - The only way you can is a butterfly kick – a snake, or wave
like undulation in the body and legs