I feel the term (keeping your palm to the sky) is more of a concentration tool more than a mechanical one. It also varies with different grips. I believe sidewinder is using a power grip (pads of the fingers on the rim) which tends to have you palm facing more perpendicular to the horizon vs a grip which has the pads of your fingers more on the flight plate (palm facing more skyward.) The key is to keep your palm facing the same direction when the disc rotates from your wrist being open and when it closes, forcing the disc to eject. Most OAT is caused by your wrist rolling over at this moment (the hit.) Concentrating on keeping your palm facing the sky helps you physically keep your wrist, and therefore your palm, in the same plane throughout the hit.