From SmashWiki, the Super Smash Bros. wiki
When used, Diddy activates a silver variation of his Rocketbarrel Pack and dashes across the screen in multiple directions, dealing damage to whoever he punches. After a while, he will lock on to the opponent that has taken the most damage, and unleash one final punch that launches the targeted opponent. After this last attack, he descends back to his original position with his Rocketbarrel Pack, which explodes once he deactivates it before he touches the ground. The explosion can damage nearby opponents.
Much like his up special and previous Final Smash, this attack takes inspiration from the rocketbarrel jetpack power-up from Donkey Kong 64, where Diddy could use the jetpack to fly around in midair.