Once you press side B, Luigi will start to charge. Once you release, he'll be launched to a certain degree depending on how long you kept the button down. There's a 12.5% chance that the attack will backfire and shoot Luigi off of the stage, so it's highly suggested that you be careful when performing this attack, and to do so rarely on small stages (more in Melee, in Brawl the missfire shoots the distance of about the fully charged non-missfire).
Mario Sports Mix
In Mario Sports Mix, Luigi will sometimes do something that looks familiar to the green missile. In dodgeball, if the dodge ball bounces off of some one, hit 1 and Luigi will try to catch it using this move. In volleyball, it is Luigi's dive, press 2 when the ball is far away from you when it's on your side.