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When the wielder is hit by a Dark-type move, its Attack is raised one stage. An Attack boost will be granted for each hit of a multi-hit attack.
Getting a free Attack boost by simply switching into a Dark-type attack seems like an awesome ability to have at first glance. However, the conditions are extremely situational, not to mention the fact that Dark-type attacks are relatively rare in most of the tiers (with the exception of Pursuit, but your Pokemon with Justified can't switch into that). Luckily, most of the Pokemon that receive Justified can make good use of a free Attack boost and have little problem switching in on most Dark-type attacks. Justified should really only be used if no better or consistent ability is available.
Found in Dream World