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The Absorb Bulb boosts the holder's Special Attack by +1 when it is hit by a Water-type move, and is consumed upon use. Does not grant immunity to Water-type moves. If held by a Pokemon with Water Absorb, Storm Drain, or Dry Skin, the ability will activate first if a Water-type move is used, so the Absorb Bulb's effect will not activate, unless the holder's ability is changed.
In singles, Absorb Bulb is somewhat useless when compared to items such as Life Orb and Choice Specs. It is essentially a one-time Storm Drain (without the immunity to Water-type attacks), but every Pokemon that could use it would much rather have Life Orb or Choice Specs, as they provide a far more consistent power boost that is not lost through switching out and does not require an activation cost.
However, in double battles, this item takes on a whole new lease on life. With the help of a friendly Surf inflicting damage on your own partner, Absorb Bulb is much easier to activate. On top of this, Absorb Bulb's quick, flexible boost is of much greater value in doubles metagames, where the switching required with use of Choice Specs is unfavorable. The item's poster child is Ludicolo, who takes nearly any partner's Surf with ease thanks to a quadruple resistance to Water-type attacks and can dish out powerful hits with the Special Attack boost.
Battle Subway, 32 BP.