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When the holder uses an attack that is super effective on the target, the damage is increased by 20%.
Expert Belt is normally held by Pokemon who either want to bluff Choice items or add an extra kick to their attacks without the recoil of Life Orb (or both!). Fast sweepers with varied attacking types greatly benefit from Expert Belt.
Some specific Pokemon that benefit from holding Expert Belt include Jirachi, Salamence, Tyranitar, Machamp, and Infernape. These Pokemon usually carry attacks that can hit common foes for super effective damage. A disadvantage of using Expert Belt over Life Orb is the decrease in damage. This decrease can sometimes put you at risk of not quite making necessary OHKOs and 2HKOs. Also, using Expert Belt means you need to forgo the boosts of a Choice item or the healing provided by Leftovers.
When using Expert Belt, it is usually recommended to use all four moveslots for attacks. This way, your Pokemon can have the widest range of types it can hit for super effective damage possible.