It doubles attack and special attack when hit by a super-effective move.
Other Stat Items for Specific Pokemon:
Deep Sea Scale - Doubles Clamperl's Special Defense
Deep Sea Tooth - Doubles Clamperl's Special Attack
Light Ball - Doubles Pikachu's Special Attack and Attack
Quick Powder - Doubles an untransformed Ditto's speed.
Thick Club - Doubles Cubone's and Marowak's Attack.
Source: Experience & this (thanks, LolAlex =3)