The effectiveness of a swimmer's freestyle or flutter kick is impacted by ankle flexibility, with stiff ankles related to a lower degree of propulsive effect from a swimmer's kick. I call this stiff ankle kick challenge a runner's kick, and I have seen swimmers, using a kick board during a kick set, move backward when using flutter kick (not on purpose). Every one of these swimmers had very stiff ankles. Put flippers on them and they were OK. No flippers, their kick was more of a hindrance thana help. My goal with these swimmers is to get their kick out of the way, keep it small and hidden behind the rest of their body via frequent pull sets using an ankle band.
If you do have an effective kick, one that moves you forward, you have to determine how often to kick or what rhythm to use when you kick. For short, sprint efforts, a 6-beat kick is common. For longer or easier efforts, the kick can vary a lot between swimmers or for one swimmer during different stages of the swim. The key is there is a limit to how much you can kick to move you forward. A good freestyle swimmer needs both parts, but many freestyle swimmers are still good without an amazing kick, and many freestyle swimmers are limited by their kick; they cannot swim any faster than their maximum kick rate. Those swimmers need to work on strengthening their pull, possibly though pull sets, focused sets with less kick while swimming freestyle, etc.
The other part of the body that moves a swimmer through the water is... wait for it... the arms. Pulling. What's the best way for a freestyle swimmer to pull, to maximize forward movement. I am not talking about body roll, high shoulders, hip roll, or any other factor that is involved - and they are part of it - but this is just about the arms. Let your body react to how you pull (note - you do want to keep your shoulders higher - further from the pool bottom - than your elbows while your arm is in the water or you will probably have some shoulder pain problems ala swimmer's shoulder; when your arm is above the water, then it stays on the chest side of your body vs. the back side; it stays lower than an imaginary line drawn through your shoulders).
What part of your arms do the pulling? What part of your arms catches hold of the water? If all goes right, from the tip of your fingers to your elbow. If you can imagine that you are grabbing hold of the water by your wrist, then you are probably going to be able to get the right image.
Your arm enters the water. You leave your elbow near the surface and let your hand, wrist and forearm drop so your fingertips and forearm point toward the bottom of the pool. You engage your lats and pecs to move your arm toward your waist. Your elbow moves in, toward your side, not in an arc. That is the freestyle pull!
Next time you go to the pool for your swim workout, think about how you are pulling. Drop into the pull, feel water pressure on your wrist, push the water back toward your waist and feet, and repeat.