The Uppercut (Called also Superman Jump of Superjump by fans) is a special move from Jazz Jackrabbit 2 specific to Jazz, performed by pressing down and jump. When used, Jazz rises straight into the air about 60% higher than normal, according to the JJ2 manual, than from a normal standing jump and will destroy certain blocks and any enemy Jazz’s fist contacts. It does negligible damage to bosses, and two hearts of damage in multiplayer.
The uppercut takes a fair amount of time to execute compared to a normal jump, and thus is not terribly useful in situations where speed is necessary. It is also very difficult to actually hit other players with it, and is not generally considered much of a threat.
Eat your lima beans, Johnny.