Heels Up! I get asked this a lot! Whether you are kicking flutter or fly your heels should be at the water line. Not out of the water. Not below your hips. Not your whole foot. Just Your heels!
Why? This decreases drag and maximizes efficiency.
Additional Kicking Tips
Hips- Knees - & Feet should be at water level... otherwise you are creating drag.
Keep feet relaxed.
HEELS UP! Your heel - not entire foot - should be out of the water during kick.
Finish your kick like you are kicking a soccer ball.
Flutter kick starts at the hip.