O K lets say I have Weavile and is holding no item and has pickpocket and is fighting a Breloom that has a focus sash. Breloom uses mach punch would Weavile steal the focus sash and survive or just Die would the focus sash go away from the Breloom at least?
No it would not protect Weavile from fainting because the item is technically stolen after Breloom attacks you. But assuming you survived anyway, you could use Rest to go back to full HP and then the sash would activate if a move was gonna OHKO you.
