shinfin fins actively encourage stroke balance for your arms and legs. They help you coordinate the smooth kicks needed to balance smooth arm pulls. So they give you a naturally balanced swimming stroke, like swimming without fins with more speed the harder you kick.
Regular foot fins unbalance your swimming style. They make your kick thrust fluctuate during each kick rather than giving a smooth thrust. This makes your arm stroke power fluctuate too. So much so, that after you take off foot flippers it takes you a while to get back to your normal balanced swimming. Good swimmers report this. This tells you that foot flippers fins degrade your kicking style. And that degrades your arm pulls too. So foot flippers make your entire swimming style worse.
shinfin fins overcome these problems. Because they attach to your legs, they actively coordinate and strengthen a naturally balanced swimming style throughout your whole body. This works best for freestyle, backstroke and fly. They even guide you into a smooth, balanced swimming style if that is new to you.
Swimming with shinfin fins feels like swimming without fins, but faster when you kick harder. You still feel the good stroke balance between your arm pulls and leg kicks. For example, doing freestyle, when you pull with your right arm you kick harder with your left leg. You need to do this to balance the forces diagonally across your body. Because your arm pulls must be smooth for good swimming, your kicks must be smooth to coordinate with your arms. That is why these fins give you a balanced, coordinated stroke for your arms as well as your legs.
Now you can have fins for extra speed and coordinated stroke balance for your arms and legs!