If the Pokémon holding a Focus Band is hit by a damagingmove 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. Focus Band can allow the holder to survive Future Sight. It cannot activate as a result of self-inflicted confusion damage.
Generations III onward
The Focus Band now has a 10% chance to activate. Focus Band now allows its holder to endure self-inflicted confusion damage.
Focus Band can allow the holder to survive Doom Desire.
In Pokémon UNITE, the Focus Band is a held item that recovers user's HP for 3s when below 25%; specifically by 8/11/14% of missing HP with 100/90/80s cooldown at Grade 1, 10, and 20 respectively. It also increases the user's Defense and Sp. Defense: by 2 and 0 respectively at Grade 1 and by 30 each at Grade 30.