Mario throws a fireball, which then bounces along the ground. A fairly standard projectile.
Up + B
Super Jump Punch
Mario uppercuts an enemy, hitting multiple times. Good for recovery and clearing out wireframes, bad for any real use.
Down + B
Mario Tornado
Mario spins around rapidly, sucking enemies in and sending them up with the final hit. Rapidly pressing B during the tornado will cause Mario to gain air, however, the recovery time afterwards makes it impractical to use to get back on stage.
Forward + B
Cape
Mario swings his cape in front of him. During the majority of the move, projectiles that hit him will instead be reflected. If an enemy is hit with the sheet, they will reverse direction; Good for ledge-guarding against many different up+B moves. It will add power to whatever projectile it reflects, and as a result, can actually be broken like a shield if it tries to reflect something that has already been reflected too much. The first time this is used mid-air, Mario will gain some air
