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The kicking dynamics are tricky…different on the surface than underwater. Dolphin and flutter kick are faster on the back underwater, yet slower on the surface. The reason is because a swimmer cannot play off the vortex from the up kick on the surface, as one can underwater. Using the up kick on the free kick on the surface is important, but the peak acceleration occurs just after the two feet pass each other. Most of the propulsion is derived by the foot moving down, not up.
Don’t overthink the up kick but rather concentrate on keeping the kick tight and holding water with your feet. Hope this helps.