A good freestyle kick creates propulsion by moving lots of water with minimal effort. A good freestyle kick feels very different to the way it looks.
Freestyle kick should move from the quads (thigh), holding the leg extended. As the quads evenly pass, up and down, there is a slight bend in the knee. You should not feel the knee bend; rather, you should feel the leg is extended. The knee will naturally bend as you feel the pressure of the water caused by the quads evenly passing. Relax the ankles, so your kick feels long and narrow.