The butterfly kick is a move borrowed from wushu. It has no flip or twist rotation. You lift your body into the horizontal plane while both legs are being swung around.
01The Dip
The easiest thing you can do to start out with the Butterfly Kick is to do a really crappy one. Just dip down with your body from one side to another and look behind you. Then bring one leg up at a time. Keep your arms open. That's it.
02Increase the lift
Now try to get your legs higher and more horizontal to the ground. Lift your back leg up until it's parallel to the ground and then jump off. Your other leg will follow.
03Butterfly Kick
Next up add the momentum steps to the setup and do a real Butterfly Kick. Remember to dip your body down from one side to the other, so you'll be horizontal.
04Cleaning it up
Last step: straighten out your legs and ideally point your toes. As you see this kind of Butterfly Kick can easily be turned into an aerial.
Tips
If you are losing balance after the landing, try to do it more upright first. Work your way down until your upper body is horizontal, as you get more comfortable with the trick.