If you've ever stabbed a bucket of water with your hands (such as spinning with some bath salt), you will undoubtedly notice that water can resist anything that passes through it-you can feel the water \"push back\" your hands.
It is this pushback that provides most of the propulsion generated by moving the fins. Just as the sole of your foot encounters resistance while swimming (thus generating propulsion), so does the fin encountering resistance and generating propulsion. But fins work better than barefoot because they have a larger surface area (so more water is encountered for that critical resistance), and-on most modern fins-the undulating motion of the fins moves in two Resistance is provided during the cycle (ie, upward motion power stroke and downward motion return stroke when the fins move through the water like a shovel.
Physics indicates that your direction of travel is exactly the opposite of the direction you pushed the water. Because of this, in recent years, fin manufacturers have attracted a lot of attention-how fins focus on the direction of water movement. Many manufacturers also focus on engineered fins with extremely flexible web sections, split fin structures and other advantages. All of these are designed to reduce the energy required to pass an inefficient return stroke during a kick cycle, while maintaining a large amount of effectiveness on the power stroke.