I once heard a story about a famous Italian volleyball coach named Carmelo Pittera. He was known as a great theoretician and technician (and was a finallist at the World Championships in 1978, so not a bad practician either). He was apparently an expert in the ‘bagher’, as the forearm pass is known throughout the non English speaking world, an invented an elaborate contraception connecting the arms and legs that forced the wearer to perform the technique ‘correctly’. As I heard the story Pittera’s greatest student was Damiano Pippi, who was a great receiver with ‘perfect’ technique and won multiple titles throughout his career, including a silver medal at the Athens Olympics.
I’ve imagined a few times what this contraption might have looked like. I guess it looked something like, but not exactly, this.
I don’t know if these kinds of things work well, but in the right circumstances I’m sure I’d be willing to test it out.