Leg fins work the large muscles in your thighs and buttocks. They also work the core muscles in your stomach and torso to stabilise your hip and back (i.e. the core muscles that Pilates exercises focus on). So these fins encourage you to use and strengthen all these muscles.
Foot fins do not use the bigger muscles in your thighs and buttocks so much. Instead, they make you bend your knees too much and work your calves. You can feel your calves and ankles doing most of the kicking with foot flippers. Your ankles get strained and your calves get tired, often cramping up.
Leg fins overcome these problems because they are attached to your legs. They encourage a proper kick from your hips with only a little knee-bend, similar to good barefoot swimming. You use your thighs and buttocks to power your hip joints, supported by your stomach and torso. So these fins give you extra speed powered mainly by your thighs and buttocks. And they tone up your torso and stomach too. Swimming with them strengthens all these muscles.
You still use your calves to bend your knees a little bit with leg fins, but much less than with foot flippers. Calf strain and cramping are much reduced. Your main kick power does not come from knee-bend. Your main kick power comes from your hips.
Now you can have fins that give you extra speed and use the large muscles in your thighs, buttocks, stomach and torso , just like good barefoot swimming!
See all FAQ about why hip power fins are the best for swimming.