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Deals damage. If the move successfully makes contact with the target, then that target's held item is removed for the remainder of the battle. If Knock Off triggers an item effect (eg. Colbur Berry) then that item's effect activates before the item is Knocked Off. Knock Off cannot remove a Griseous Orb from Giratina-O, a Plate item from a Pokemon with Multitype, or a Drive from Genesect, but it can remove a Griseous Orb from any other Pokemon, any item other than a Plate from a Pokemon with Multitype, and a Drive from any other Pokemon. Pokemon with Sticky Hold cannot lose their held items. Items lost to this move cannot be regained with Recycle. In-game, after a battle, the Pokemon recovers its held item. If Exp. Share was Knocked Off in battle, the Pokemon holding it still gains experience as though it had never been Knocked Off.
Knock Off is a utility move; it's there simply to get rid of whatever item the foe may be holding, something that is threatening to almost every Pokemon around in competitive matches. Most walls, if they lose their recovery item, become far easier to take down, while most sweepers will be crippled if they lose their boosting items. Knock Off is particularly threatening to Eviolite users, since without the defensive boosts they become (most of the time) on the verge of being useless. The fact that it does damage means it can't be stopped by Taunt. Not bad at all. The only difficulty is finding a Pokemon that can afford to give up a moveslot for it.
Trick is a non-damaging move that switches the items of the user and the target with each other.