From SmashWiki, the Super Smash Bros. wiki
The boomerang superjump is a superjump glitch that can be performed by either Link or Young Link in Super Smash Bros. Melee. It is done by throwing their boomerang, and then wall-grappling on a wall or the side of a stage. If performed correctly, the boomerang will fly into Link/Young Link and they will immediately fly high up into the air, in a tumble animation. If anything is done to disrupt the tumble animation (jumping, air dodge, etc.) then Link/Young Link will cease to rise into the air. This will happen automatically after a certain period of time, but disrupting it will cut it short.
This glitch is part of another technique, the chain dance.