Regular foot fins make you bend your knees too much. They sink your hips and thighs too low in the water. So they slope your body down in the water too. Then the water flow hitting the front of your body and thighs causes too much drag and wastes your energy. Foot flippers sloping your body and thighs downwards, causing drag, is a very common problem.
In fact, even for swimming without fins, many people (other than good swimmers) still have their body sloping down in the water. This increases drag and makes it much harder and less enjoyable to swim. If you feel like you are poorly streamlined and wasting energy, then your body is probably sloping down in the water.
Now you can have fins that lift your body horizontal and streamlined for less drag, higher efficiency and more fun swimming!
See related faq about how these fins give you a smooth streamlined kick.
See all faqs about why these are the best swim style fins.