Leg fins lift your thighs horizontal for a more streamlined kick. This reduces thigh drag and increases the thrust from your kick. So they give you a faster streamlined kick for freestyle, backstroke and fly.
Because regular foot fins are on your feet, they slope your thighs down in the water when you swim. You feel the extra drag from the water pushing against your sloping thighs. This thigh drag wastes your energy. So foot flippers don’t help to lift your thighs to streamline your kick. Quite the opposite. Foot flippers sink your thighs and work with different kick to good swimming without fins.
Leg fins overcome these problems because they attach to your legs. The water flowing over the fins pushes up on your shins, which lifts your knees. So this lifts your thighs horizontal and streamlined. Most people feel this as soon as they swim with them.
Many swimmers know they should have their thighs horizontal. But still their thighs keep sloping down when they kick. Wearing leg fins helps you to get rid of the bad habit of sloping your thighs down in the water. The trick is to relax and forget about your old kick. It often helps not to kick at all to start with. Just get some speed from your arms and let the fins lift and straighten your knees. Then add a gentle kick and let the fins guide you into a streamlined kick from your hips. You will feel your kick power coming from the larger muscles in your thighs, buttocks and torso. Build your kick from there. With practice, you will lose your bad habit and your body will remember this new streamlined kick instead.
It helps if you already have enough hip joint movement range to straighten your thighs, at least in line with your upper body. If not, then doing stretches on land can help to increase your range of hip joint movement. Swimming with leg fins will also help to improve your range of hip movement.
Now you can have fins for extra speed and have your thighs horizontal for streamlined swimming!
See related FAQ about higher straighter knees and higher straighter hips.
See all FAQ about smooth forces for faster swimming.