(And not collapse the distal knuckle of your middle finger.)
- Your left hand knuckles will need to be slightly more horizontal and your right hand knuckles slightly more vertical. Not much, but a titch.
- Keep the long bones of your hand aligned with the long bones of your arm. This will allow the tendons to function without impediment. Breaking that line is fatal (on both harp and piano). Given #1, this is why you need to keep your elbows up.
- When you reach out with the ring finger, keep it curved or else the middle finger knuckle will lose every ounce of firmness and collapse. Those two fingers share a tendon; they either curve together or they collapse together. To keep the middle finger curved and firm at the knuckle, the ring finger must be curved as well.
All of these things work together. A year after getting my harp, I can do four-fingered turnarounds (although not quickly) without pain or difficulty. These three points made it possible, and they took me a year of stubborn analysis to work out.