When smash thrown, it gives very powerful knockback, almost always resulting in a KO. When a player touches an activated bumper, they get bounced with much force in the direction away from the bumper. In the original Smash Brothers game, the bumper would skid a distance in the direction opposite. It does not skid like this in Brawl, and remains stationary when activated until it disappears. It is also one of the most useful items in Multi-Man Brawl.
In Super Smash Bros.
Tilt Up: 7%
Tilt Down: 1%
Smash Up: 9%
Smash Down: 1%
Tilt Up: 9%
Tilt Down: 9%
Smash Up: 9%
Smash Down: 9%
In Super Smash Bros. Melee
The Bumper item was removed in Melee.
In Super Smash Bros. Brawl
The Bumper item returns as an item.
It appears as a trophy in Brawl. It is received by hitting a total of 15,000 feet with all fighters Home-Run Contest records.
An item that originally appeared in Super Smash Bros. on the Nintendo 64. After many years, it now makes its triumphant return. Bumpers are used in pinball to send balls flying, and they perform the same role in Smash Bros. They react to anything that touches them, including the people who set them. As they can also be set in midair, using them requires good tactics.
In Super Smash Bros. for Wii U / 3DS
Ah, pinball... What a fun game. And with this item, you can add a bit of pinball wham to the battlefield. Once thrown, this item will bounce anything that comes near it violently away. Just don't get careless and walk into a deployed one yourself, or you'll get bounced too!
In Super Smash Bros., if a Fox player smash up throws a Bumper on the tower on the right of Hyrule Castle and then uses Reflector, the bumper will hit the reflector five times, break it, and then deal large amounts of damage with low knockback, often leaving Fox on 999% damage. It is also possible to do it with two Bumpers by smash up throwing the first one and then normal up throwing the second one. Another stage this can be done on is Mushroom Kingdom. 
Despite not being an item on Melee, being replaced by the Flipper, the Bumper makes an appearance in Collect the Trophies in Classic Mode, being two giant parts to each side of the stage.
When thrown on a platform that disappears or shrinks, the Bumper will bounce if the platform does either.