When Focus Sash's definition says that it can only be used once it means only once PER BATTLE. Focus Sashs will not disappear after only one use ^_^ Example: You have a Shedinja holding a Focus Sash In-game (which is common and popular). You get hit by Crunch. HP does not go down due to 1 HP and Focus Sash. Focus Sash remans equipped, but WILL NOT BE ACTIVATED AGAIN, even if you switch out.