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If the holder has max HP, Focus Sash guarantees they survive any attack with 1 HP. 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 all hits of a multi-hit move like Fury Attack, provided the holder is at 100% health on the first hit. Also allows the holder to survive all hits of Triple Kick. Focus Sash prevents the holder from OHKOing itself in its confusion. Does not protect against Doom Desire or Future Sight. Does not protect the holder from OHKOing itself from recoil damage. One-time use.
Because of omnipresent entry hazards immediately nullifying a Focus Sash, it is seldom used on non-lead Pokemon. However, this does not make the Focus Sash a bad item by any means. Focus Sash makes fragile suicide leads such as Azelf and Aerodactyl much more potent. Focus Sash allows these suicide leads to set up Stealth Rock (or other entry hazards) much more often than they could if they held a different item. Focus Sash is also useful on Endeavor-using Pokemon like Mamoswine, Smeargle, and level 2 Clefable. Lastly, Focus Sash can buy an extra turn for sleep-inducing leads like Gengar, Roserade, and Smeargle.