Keeping the body on the surface during all the hustle of butterfly is the first step. Practicing a good amplitude without going vertical or dolphin diving. Don't worry about timing yet.
Timing is everything is butterfly! Focus on BPG (breathe, pull, glide) timing and the arms and head will all move at the right pace. Practice in the shallow end without moving at first.
Teaching butterfly is tough because there aren't many stepping stones between drill and swimming. 3k1p (3 kicks 1 pull) helps us swim the stroke, but slower and easier to put together the body position and timing.
Butterfly requires rhythm, and 333 helps set the rhythm with single strokes and hold it on the full strokes. Don't use this as a replacement for real butterfly!
We want the stroke to return to as much as a streamlined position as possible during every stroke. F2S (fly to streamline) drill helps force kids to return to the streamline every stroke.
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By swimming butterfly with freestyle feet, you can practice increasing the arm tempo if kids tend to be dolphin diving and taking a break in between strokes.
Thumb drag drill can help teach kids to recover their arms low to the water as they try to drag their thumbs through the surface.
When the arms land in the water, they should land high and reach forward, not down. Connecting the wrists over the water helps teach that.