Butterfly kick timing is often described in terms of what the arms...

Kick when the hands enter.
Kick when the hands exit.
While that’s accurate, it can be confusing.
Some swimmers are surprisingly unaware of where their hands are in space.
When the head is moving down, it’s time to kick as the hands approach entry.
When the head is moving up to breathe, it’s time to kick as the hands pull through.
It’s simpler, it’s more intuitive, and it can be more effective.
If swimmers are struggling, give it a try.
The more swimmers improve, the more the system needs to be updated.
When they get faster, the standards and expectations need to rise.
And that cycle never stops.