The butterfly leg action is undulating. We call it dolphin kicking or mermaid/man kicking when we teach children. It begins from the chest down, and helps propel the body through the water in streamline position. This video shows a massive sine curve and wave fo the leg action but notice how the toes are together and the ankles kick out when the downstroke is done.
Also worth noting is that there is an "up-kick" and "down-kick" to the butterfly leg action. The best way to develop a feel for the leg action is to do the kick on your side. This helps build that muscle memory and wiring in the motor cortex of the brain so that you can butterfly kick with ease.
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