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When the wielder consumes an item or has one removed from it (for example via Knock Off, Bug Bite, Fling, or Natural Gift) its Speed is doubled until it switches out. This boost is not a change to the wielder's Speed stage, so if it were to use Agility after Unburden activates, it would have four times its original Speed. The Unburden boost cannot be Baton Passed, and losing items after the first does not provide additional boosts. Unburden will not activate if the wielder simply has no item; an item must be removed for it to activate.
As Unburden is only available on a few viable Pokemon, its competitive use is limited. Sceptile is probably the main user and can take advantage of it to get a further boost out of item and ability combinations like Flying Gem and Acrobatics. Otherwise, Unburden is barely seen, as all the other Pokemon with the ability are not fit for competitive OU battles, and the Speed boost cannot be Baton Passed.
Found in Dream World