Quirky Momentum Question?

What would happen if someone had a device that reversed their momentum(Complete and instantaneous) and jumped in the air activating it on the way down? I'm assuming they would "bounce" into the air a short distance before gravity pulled them down. Also if this mythological device reversed momentum and maintained your reversed momentum (Still dropping from a jump), would it negate gravity and send you flying into the sky or would it cancel out gravity and cause you to hover? Assuming a normal standing jump (and average 150 pound male) and not a leap from a building or high object.