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If the attack would have KOed the holder, there is a 10% chance that it will instead survive with 1 HP. Focus Band will allow the holder to survive all hits of a multi-hit move like Fury Attack. Also allows the holder to survive all hits of Triple Kick. Focus Band 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.
While it may seem useful at first, Focus Band is not a good item. Since it is so unreliable, the risk is too high for the payoff. Even then, the popularity of Tyranitar and it's Sand Stream ability makes it even worse, since sandstorm will make the surviving Pokemon faint. It is also largely outclassed by Focus Sash, even if it only works when the user has all its HP, the activation ratio is 100%, making it much more reliable.