The Focus Sash (Japanese: きあいのタスキ Fighting Spirit Sash) is a type of held item introduced in Generation IV that prevents the holder from fainting in one hit.
If the holder has full HP and is hit by an attack that would otherwise cause fainting (including a one-hit knockout move), it survives with 1 HP. Can also protect against self-inflicted confusion damage. Protects from all strikes of a multi-strike move. Cannot protect from damage from Future Sight or Doom Desire. Disappears after use.
From Generation V on, it can only protect from one strike of a multi-strike move.
A Focus Sash was seen held by Darach's Houndoom in Getting the Drop on Gallade II, allowing it to survive a super-effective Focus Blast by Platinum's Lopunny. Platinum also rented a Focus Sash for her Empoleon to hold in the Battle Castle.
In Walkthrough! Battle Tower!!, Yū had given his Infernape a Focus Sash to hold, allowing it to survive a Golem's Earthquake at the Battle Tower and then defeat it with a Grass Knot.