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Doubles Ditto's Speed stat. If Ditto transforms into another Pokemon, the boosts are lost.
At first, the ability to Transform into the foe first is vital for Ditto, as it cannot withstand even the slightest of attacks thanks to its miserable defenses. However, because this boost is lost once Ditto transforms, Quick Powder is rather useless, even if you happen to be using Ditto in the first place. Ditto should plan on using items that will be useful even after transforming—or better yet, use its Dream World ability, Imposter, with which point it transforms automatically.
Wild Ditto (50%).