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If the holders current species is Pikachu, its Attack and Special Attack is doubled.
Light Ball is the item which makes Pikachu usable, boosting its pitiful offensive stats to impressive heights. Pikachu with 152 Attack EVs reaches 368 Attack, and the special sweeper (252 SpA) hits 398 Special Attack.
Light Ball gives Pikachu impressive offenses for a mixed sweeper, even when its EVs are split. In fact, Light Ball is the only item really usable on Pikachu in basically any tier, as Pikachu has hideous base stats in everything except Speed. It lacks the recoil of Life Orb, boosts both stats much more than Choice items would, and leaves it with the freedom to switch moves or use Encore / Substitute.
Unfortunately, Pikachu has decent Speed but not quite enough to pull off a sweep most of the time, something Light Ball cannot remedy. Pikachu is also useless without Light Ball, so Knock Off or Trick render it dead weight on any team with it.
It is also worth noting that while Pikachu hits very hard with Light Ball Volt Tackle in particular, the recoil will really hurt it, especially considering its pathetic HP. Pikachu can suicide attack Blissey, dying alongside it after Volt Tackling with it.