Everytime a fin starts or stops, there is a water vortex associated with it. The aerodynamicists call it a starting or stopping vortex. It has to do with boundary layers building up or being shed. The stopping vortices are bigger, faster, stronger. The vortex is just water spinning, and the energy in that vortex is energy wasted. That wasted energy comes from your legs. So, traditional fins waste less energy with broad, sweeping motions, so they start and stop moving as little as possible. Split fins are designed so the blade separates and bends such that a vortex is shed off each of the half-blades, and the geometry makes those vortices in the middle "slot" spin in opposite directions, which propels water between the two vortices. This gives some propulsion. For split fins, you want lots of water being propelled, so the more often the fin changes direction, the more propulsion you get. This means rapid, small-amplitude kicks, vesus the slow, large-amplitude kicks for the blade fins. Different fin design>>different optimal kicking. There is also likely a venturi effect with water moving over the split-fin blade being "squeezed" to get through the slot, and this gives some more addeded momentum forward. All th above is for flutter kickers. Actually, many flutter kickers are actually trying to ride a bike underwater, with lots of knee bend and thigh movement. They get some minor propulsion from pushing the water with the bottom of their fin, whereas fins are designed for the forward kick of the foot to be the power stroke, utilizing the top of the fin.
