Marth's hidden move
This is very hard to do so go to training mode to try it first. Pick Marth as your character and any other one as your opponent. Use his Left/Right + B attack. Hold your joystick on the nunchuk facing your opponent and keep clicking B. Right before the last attack of that combo is completed, move the joystick Up or Down. Moving it Up will throw the opponent up. Moving it down will make Marth crouch down and stab the other guy alot of times. Remember, this is very had to do, so be patient. I found this out by accident when I was battling Ike with Marth in traing mode.
More tricks, tips and cheats for this game are right here - Super Smash Bros. Brawl cheats