Kicking on back (also called backstroke kick)
The mechanics are largely the same as doing flutter kick on your stomach with the main difference being slightly more hamstring recruitment. If you kick hard on your back and use your quads to finish each kick, that will continually drive the legs lower and lower in the water. When on the back the hamstrings are recruited slightly more to work the down stroke of the kick to push water down to keep the legs up in the water. Don’t think about using your hamstrings on purpose though. Just kick on your back and keep your knees near the surface and let the feet be just under the surface “boiling” the water.