One quick rule. You jump by pushing your body away from the earth with your legs and hopefully get enough momentum built up to keep going after your legs can't reach the ground any more. The fastest the body will move will be the last instant that the legs can reach the ground to push. After that, there's no push and gravity immediately begins to slow you down. If the body moves faster up after leaving the ground than it ever moved during the push... something is wrong. It can't do that.
Damn near impossible, except for birds and flying saucers.