Open heel fins are fins you use with boots. There’s a number of different types to choose from. Solid rubber fins take strength to wield but are great in currents and easy to maneuver with (going backwards and doing helicopter turns). Plastic fins are light to pack and take less strength, but give less power. Split fins are the easiest to move through the water, but don’t propel you forward as effectively, and maneuvering on fin power is very hard.