Question: what is the difference between focus sash and focus band?
Answer: Focus Sash is a one time item that only works when you are at full HP. Focus Band has only a 10% chance of working (rather than Sash's 100% chance) but you don't need to be at full HP in order for it to work (think of it like Endure). Both allow your Pokemon to survive what would have otherwise been a KO with 1 HP.