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If the holder has max HP, Focus Sash guarantees it survives any attack with 1 HP. The item is consumed after use. If the holder is not at full health, the item will not activate. If the holder takes damage, but does not activate Focus Sash, and then recovers off this damage to return to 100% HP, Focus Sash can still activate. Focus Sash will allow the holder to survive the first hit of a multi-hit move like Fury Attack, but it will be KOed on the second. Focus Sash prevents the holder from OHKOing itself with confusion damage. Will activate if the holder would be KOed by Final Gambit, but does not protect against Doom Desire, Future Sight, or Destiny Bond. Does not protect the holder from OHKOing itself with recoil damage. Focus Sash will activate before Knock Off, so if Knock Off would KO in one hit, then Focus Sash will activate first and prevent the KO. A Pokemon with Sturdy will have its ability activate before Focus Sash, so Focus Sash has no effect on a Pokemon with Sturdy unless facing an opponent with Mold Breaker.
Focus Sash is commonly seen on frail leading Pokemon to guarantee they survive at least one attack. Often, this allows the Pokemon to set up in some way, whether by distributing entry hazards, setting up dual screens, or boosting stats. Its utility declines significantly for Pokemon that are not leads, as Stealth Rock, Spikes, and sandstorm are all very common. A notable exception to this rule is Alakazam, which, thanks to Magic Guard, is immune to all passive damage, meaning that with Focus Sash, it can act as a perfect revenge killer if the situation demands it. It is also a very useful item for Shedinja, who can use it to survive any attack once.
Battle Subway, 48 BP.