From the Super Mario Wiki
Diffusion Beam is R.O.B.'s Final Smash in Super Smash Bros. Brawl. R.O.B. shoots a constant beam from its eyes for the duration of the move that deals continuous damage to any nearby opponents and will occasionally shoot a blast of energy. R.O.B. is able to move freely, is invincible to damage and unaffected by knockback, and can use any of its other moves while this move is in use.