From the Super Mario Wiki
Mario Tornado is Mario's down special move in Super Smash Bros. and Super Smash Bros. Melee. When this move is used, Mario spins rapidly to hit opponents multiple times and finishes with a full-body stretch that deals a single, high-knockback hit which can meteor smash in Super Smash Bros. If the special button is pressed rapidly, Mario can rise up in the air slightly. Luigi's down special, Luigi Cyclone and Dr. Mario's down special, Dr. Tornado are based on this move.
Since Super Smash Bros. Brawl, F.L.U.D.D. has replaced the Mario Tornado as Mario's down special, though this move was still kept as Mario's down aerial move. As a result, Mario loses a recovery option, but Mario Tornado's hits link better, and the final hit inflicts greater knockback.