The Foil Monofin from Finis is a soft, rubber swim fin that both feet fit into so that you can learn or improve your dolphin kick. This single blade fin has an open heel and toe design to fit a wide range of foot sizes and is ideal for all ages and swimming abilities. Ideal for learning and improving the dolphin kick for butterfly, this fabulous fin will also work your core and leg muscles and will automatically develop the undulating Foil Body Motion needed to help you benefit the most from a dolphin kick. The Foil Monofin is smaller than the Rapid Monofin and ideal for swimmers. Using this monofin will also increase the flexibility of your lower back due to the development of the undulation movement caused when you wear a monofin.