@AndrewKSheaff: I think a lot of the emphasis ...

I think a lot of the emphasis on undulation in butterfly and breaststroke is misplaced.

Undulation should occur because of what the arms and legs are doing, NOT the other way around.

That has big implications for how to improve skills.

And just like rotation TIMING matters in freestyle and backstroke, undulation TIMING matters in breaststroke and butterfly.

If the right things aren’t happening when the body is moving up and down, races get a lot longer and a lot slower.

If I want to help swimmers learn to create a lot of propulsion, I combine performance strategies and sensory strategies.

Swimming with a parachute and a closed fist is going to help them FEEL the forearm.