I used to have a severe problem with cramping, especially after fighting a current.
My first solution was splits. Nice, for a RAPID, high-frequency narrow flutter. That lasted 2 months and I went back to the Jet Fins.
I now almost exclusively frog kick. It is actually much easier on my legs, results in no cramping, and has reduced my air consumption dramatically, as I am no longer using the large thigh muscles except in a strong current.
Using little more than my ankles and lower legs (and my ankles are not strong) I can now frog as fast as I used to flutter. I generally hold my knees at about a 45 degree angle (but this will vary with the pressure in my tank) and simply rotate my ankles with very little movement of the thigh. When everything is in sync, it just "clicks" and you start moving like a rocket.
The backwards kick is a variation on the frog. With the feet in the "up" position, you rotate your ankles to plant the feet sole to sole, as if in a praying position. You then thrust your feet down and out, in a reverse movement to a normal frog recovery, then splay your toes apart and pull back to the reset position.
Hard to describe, and hard to do.
I seriously doubt that it can be done efficiently with split fins.