Oh really? Stationary box you say?

So by that logic a "stationary box" coming to an immediate halt after traveling 100mph vs traveling 5mph will have the same end effect on the person inside the box? Or how about a box that is not moving at all? Relative velocities are a thing. And as I pointed out before, most of the force you are extorting in a flip is in the turn, not a straight upward motion.

Or....more specific to this situation. A backflip in an elevator traveling at a constant speed of 100mph upward should result the same as trying a backflip on the ground? What if that elevator was traveling 1000mph? What about at the speed of light? At a constant speed it is treated as a "stationary box," right?

Real world applications dont occur in a vacuum my dude.