Do you ever have challenges getting the horizontal leg to stay stationary when the other leg is opening to split?
What about the hips shifting while the leg opens to split?
Those are common errors that happen to swimmers on the pike to split transition for Surface Prawn.
Below is a link to a drill that really helps swimmers feel and see how to properly execute this transition.
The swimmer sits on deck facing away from the pool with their bum as close to the edge as possible. When one leg moves to split the other remains still and the knee faces the ceiling. The leg will move around and eventually end up over the water in a half land half water split.
I like this drill because the swimmer can see the leg in front. It also allows the swimmers anchor the leg while the other leg moves. Anchoring the leg really helps the swimmers feel the correct movement.