Fins are a powerful tool to move underwater, and their shape, weight, buoyancy, and hardness vary greatly from brand to brand. Divers typically use positive or negative buoyancy to correct their body position while diving, but few share opinions on whether the material is soft or hard.

The power generated by a single kick of a fin is clearly related to the fin's shape and the area of the fin. A larger fin surface can move a larger volume of water. The difference between hard and soft fins lies in the time it takes for propulsion to return. With soft fins, the fin surface deforms immediately after the kick is completed. The diver then waits for the fin surface to reel back, pushing the water and generating momentum, resulting in a slower return.

In other words, soft fins help divers maintain a rhythm of kicking and gliding, ensuring a smoother flow and avoiding excessive kicking that can lead to air loss. However, firmer fins, because the fin surface doesn't reel in the water, appear more streamlined and stable. In addition to low-frequency kicking, you can also use higher-frequency, faster kicking. While high-frequency kicking consumes more air, it can effectively convert it into forward momentum, meeting some urgent needs. In the following video, the yellow one is the softer Seac Propulsion Pro, and the black one is the firmer Apeks RK4. Both use the same low-frequency kicking motion. The soft fins produce a rolling motion even with small movements, while the firm fins can quickly complete even large movements.