Knock Off deals damage and if the target has a held item, damage is increased by 50% (i.e. 97.5 base power) and the item is removed for the remainder of the battle (switching out does not restore the item). All items are restored at the end of a battle. If the item would activate on attack (e.g. Focus Band), it does so before being stolen. If the item is consumable on attack (e.g. Focus Sash) then the item is consumed as normal and cannot be stolen. Pokemon with the ability Sticky Hold cannot have their held item removed. The Griseous Orb cannot be removed from Giratina; Type Plates cannot be removed from Arceus; Drives cannot be removed from Genesect; and Mega Stones cannot be removed from their respective Pokémon. Note that in these cases the damage dealt does not increase.
In Generations 3-5, Knock Off has base power 20, 20 PP, and did not increase damage when knocking off items.