Next stroke: Butterfly

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I am about to attempt a move into butterfly. My understanding is that butterfly came about from breast stroke. And so, I want to transcend into fly from the breast stroke. I can easily replace the frog kick with a dolphin kick but what I don't see is where the two kicks in fly work into the mix? Am I right one kick is stronger and one weaker. I have been told it is a rhythmic stoke, I am looking for the rhythm.