From the Super Mario Wiki
Flippers are obstacles in the game Balloon Fight. They are also seen in Super Smash Bros. Melee. It is possible that they replaced the Bumpers from the original Super Smash Bros. Players can pick it up and throw it. When thrown, it will hover in the air. Any player who comes in contact with it gets hurt, and the flipper spins around, hurting other people. After some amount of time, it disappears.
Flippers move along with the stage if it is a moving stage. This can be used by players in stages such as Big Blue to place the flipper which then hits another character who is standing on the flipper's route. In Bonus mode, there are special points awarded if an opponent is KO'd using a flipper.
 Trophy Information