Basically, you look straight down and shoot a rocket, launching you high into the air and (theoretically) into otherwise secret and unreachable parts of the map. Of course, the skill is in surviving it :P
The definition's simple, a rocket assisted jump. Yoink out the rocket launcher, aim at your feet, jump, and fire the rocket. The resulting explosion increases your intertia. Angle can be adjusted to leap in the desired direction. Simple.