Leg fins give you constant speed during each stroke, for extra swimming speed overall. This applies to your down kicks, up kicks, and to the transitions between kicks. You need constant speed throughout each stroke (i.e. within each stroke) to optimise your stroke efficiency. These fins do this best for freestyle, backstroke and butterfly.
When you swim with regular foot fins, you can feel the speed changes within each stroke. The speed increases coincide with each down kick. You can feel it and hear it as the water flows past your ears. So regular foot flippers give you a fluctuating speed kick. They do not give you the constant speed within each stroke that you need for maximum efficiency. So foot flippers mess up your entire stroke.
Leg fins do give you constant speed during each stroke because they are attached to your legs . Firstly, your legs are more horizontal and you kick from your hips with only a little knee-bend. Secondly, these fins are designed to flex smoothly towards and away from your legs during your kick. This helps you swim most efficiently, by keeping your speed as smooth as possible throughout your entire stroke. Smooth kicking helps smooth arm pulls. Both together, give you a more consistent speed during each stroke. Even more, your body remembers this constant speed kick, even when you go back to swimming without fins. So swimming with these fins improves your normal swimming too!
For efficient swimming, you need constant speed within each stroke. You achieve this with long smooth arm pulls together with smooth kicking. Listen to the water flow past your ears to measure how consistent your speed is. If your speed is constant, you will hear a smooth and steady sound. If your speed is going up and down during your stroke, you will hear a changing sound.
Now you can have fins that give you extra speed and constant speed during each stroke.
See all FAQ about why hip power fins are the best for swimming.