Let's step back and analyze the pieces of various kicks to try and demonstrate why split fins don't work well.
The following descriptions of the kick are very rough....
Frog kick:
- The kick begins with your knees bent, feet flat, legs together.
- The fins are extended sideways, while flat, with the edge of the fin cutting through the water
- The ankles are turned in, and the feet are brought together while extending the legs. A really strong frog kick can have the bottom of the fins touching at the end of this step.
- feet are brought straight back and then ankles rotated out again to the starting position
A frog kick is 'doable' in a set of split fins, though there is a lot of efficiency lost especially in the part where you separate your legs and bring the fins out. The split fins, due to their inherent floppiness and extra edge catch the water on this motion and pull the split apart, wasting a bunch of energy. Then, on the kick portion where you extend the legs, the fins go in like 8 different directions. The bigger the kick, the more this happens. Again, video does not lie.
Back kick:
A back kick is like a frog kick, in reverse.
- First, you bring the feet together, ankles turned in with the knees bent
- The feet, together, are pushed backwards, using the tips of the fin to break the water so you don't move forward in this step
- The legs are then extended and the fins flared while you bring the legs back in. This is where you move backwards
On a back kick, the part where split fins fail miserably is the extension with the fins together. The tips of the fin flail in multiple directions and tend to cause forward thrust when you are trying to move backwards.
The helicopter turn is a frog kick with one leg and a back kick with the other, so the deficiencies of the split fins should be obvious from the above description.
