Focus Sash activates anytime something would be 100% KOed by a move so I assume soIf a Volcarona came in on SR and used Roost to get it back to full HP, if it got hit by a move that would KO it, and it was holding a Sash, would it activate?
(Simpler version: If a Pokemon that took prior damage got back to full health and got to 100% HP got hit by a move that would KO it, would a Focus Sash activate?)