The explanation in SCIENCE OF SUPERMAN, which Mark Waid seems to use in BIRTHRIGHT, is that Superman's nervous system is superconductive. According to a famous Russian antigravity experiment, conditions of superconductivity allow for not only electricity to flow freely, but alters the flow of gravity as well.
Got a cite for that? I've never heard of anything being able to negate gravity, and it would pretty much revolutionize physics if it were proven true.