If the Pokémon holding a Focus Band is hit by a damaging move that would cause it to faint, there is a ~12% (30/256) chance the Focus Band will activate and cause its holder to survive with 1 HP. It cannot activate as a result of self-inflicted confusion damage or Future Sight damage.
If Focus Band activates as a result of a multi-strike move, the holder does not faint from any of the strikes.
Generations III to IV
The Focus Band now has a 10% chance to activate. Focus Band now allows its holder to endure self-inflicted confusion damage.