The user charges energy spheres in his hands and fires them one after another, inflicting a great deal of damage.
Piccolo uses this attack in his battle with one of the Cell Juniors. After Goku and Vegeta are shown to have trouble facing other Cell Juniors, Piccolo uses Rapid Fire on the one battling him, however the small Bio-Android dodges them all with ease, creating explosions where Mr. Satan was carrying Android 16's head.
This attack is later used by Cell in his battle with Super Saiyan 2 Gohan. After the Destructo Disc and Special Beam Cannon prove no match for Gohan, Cell tries Rapid Fire. However, the small energy spheres go right through the young Saiyan, much to the shock of Cell. Cell uses Rapid Fire differently than Piccolo (who fires it with both hands), firing it with one finger.