This is such baloney. Split fins are meant to use in flutter kick. Most divers paddle around shallow reef. You can back kick and heli turn with every type of fin, including forece fins and split fins.They're not conducive to frog kicking, back finning, helicopter turns, flutter kick (from the bent knees, not that scissor kick thing). The split can trap line.
Basically not good for proper finning skills and very rarely seen on boats.
I have tried several split fins and don't like tham but that doesn't mean they're crap. They're for people that need or want a low resistance fin. They work just fine.