While playing party mode, as long as it is not your characters turn, make your character cheer by pressing the "L" button. If used with the "C-stick", you can manipulate the cheer.

"L" with "C-stick" up speeds the cheer up
"L" with "C-stick" down slows the cheer
"L" with "C-stick" left or right raises the volume of the cheer

Beat the game with any character to unlock new mini-games: Beach Volleyball, Goomba Stomp, and Panel Panic.