When hit with a move that will cause the holder to faint, there is a ~12% (30/256) chance it can survive with 1 HP. This can also also protect against recoil damage and self-inflicted confusion damage. Focus Band only needs to trigger once to protect from all strikes from a multi-strike move. It cannot protect from damage from Future Sight.
Generation III
The chances of Quick Claw activating are decreased to 10%. Unlike Future Sight, Focus Band can protect the holder from Doom Desire.
Generation IV
Focus Band no longer protects the holder from Doom Desire.
Generation V onward
From Generation V on, Focus Band must trigger separately for each strike of a multi-strike move.