My DD used one for a while and said it felt better when she threw. Kept her arm "whippy", whatever that means. But was to hot to wear in Florida in the summer.
DD wears an Evoshield sleeve. Like riseball, she said is helps stay "whippy". I think it keeps the muscles warm, loose, and supported. An additional benefit, it keeps the arm from chafing.
It's just a compression sleeve, if your kid wears underarmor it accomplishes the same thing. My oldest tried a bunch of them when she was having an issue with an overaggressive "brush" that resulted in a stress fracture, but she couldn't keep them on, they would slide down and roll up around her wrist from the arm whip.
Kinesiology (K-taping) is a taping method that is supposed to gently lift the layer of skin and attached tissue covering a muscle so that blood and other body fluids can move more freely in and around that muscle.
You see this sticky elastic tape all over athletes these days.
I know that SOME brands of arm sleeves suggest they induce the same effect.
Another GREAT this about arm sleeves is if you have a pitching DD like mine that likes to dive all over the place, without arm protection she is always getting raspberries on the inside of her elbow. Try pitching effectively when yer bleeding from an inner elbow raspberry....
But we do battle keeping them from falling down too......
To keep the pitching arm warm. When it's cooler outside the outside temperature cools the arm, the sleeve helps keep it warm. My DD's college coach wouldn't let the pitchers or catchers set foot on her field in cooler temps without sleeves. She claimed cold arms diminished their abilities.