Like Rocket Jumping, the Double Jump has become a staple feature for some platforming games. In order to perform this gravity defying action, a character would have to have the ability to, at any time, instantly create a force exactly equal to their current downward force due to gravity acting at their feet. Any greater, and they'd be floating upwards. Any less, and they'd be falling slowly. Creating this force acts as a normal force, like the force the earth enacts upon us as we just walk around. Theoretically, if someone were able to do this, they may be able to push enough to "jump" in the air again. Just remember, what goes up must come down.