From the Super Mario Wiki
Rocketbarrel Boost (Boostum Highus) is a signature move of Diddy Kong allowing him to fly around with his Barrel Jet. The move first appears in Donkey Kong 64 and has been subsequently featured in the Super Smash Bros. series.
Donkey Kong 64
After acquiring the ability, Diddy is able to jump into a Diddy Barrel. By doing so, he activates his Rocketbarrel Boost ability and flies around. As with the other Kong Barrel abilities, the Rocketbarrel Boost is powered by Crystal Coconuts. Diddy is able to utilize his Peanut Popguns while performing the move. The Rocketbarrel Boost is also barred in a few sections of the game.
Super Smash Bros. series
The Rocketbarrel Boost, originally known as Rocketbarrel Blast from the Brawl manual, appears in the Super Smash Bros. series as Diddy's up-special move, starting in Super Smash Bros. Brawl By using Rocketbarrel Boost, Diddy can blast up using his Rocketbarrel Pack. The player can charge this move to blast Diddy up into the air. The longer they charge the move, the farther distance the distance that Diddy blasts. Tilting the control stick allows Diddy to curve his flight path, allowing him to fly in several different directions. Diddy Kong can ram into opponents to damage them, and if used in mid-air the initial burst of height acts as a meteor smash. However, if Diddy Kong is attacked while flying, he will lose his jetpack, and fall down. The jetpack will then fly away, and has a chance of hitting another player.
This move has a unique property to physically react when it goes into a wall (unlike most other attacks in Brawl that simply resume the animation). If Diddy collides with a wall or another character hits him, his flight will take an abrupt stop, his pack exploding and the tow rocketbarrels flying off randomly. He will receive a small amount of damage when he collides into a wall.
In Super Smash Bros. for Nintendo 3DS and Super Smash Bros. for Wii U, two custom variants can be unlocked for this move. The first one is Rocketbarrel Attack, which deals more damage when Diddy hits an opponent, but his flight path cannot be controlled as easily. The second is Rocketbarrel Kaboom, which charges faster and can be steered easier, but doesn't travel as far and explodes if it hits terrain. The Rocketbarrel Boost is also used during Diddy Kong's Final Smash, Rocketbarrel Barrage.
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