At the duelling club, Malfoy taunts Harry about how he’s forbidden to use his knives by saying something about how he can’t parry with them. It turns out that, because goblin silver "imbibes only that which strengthens it", it actually reflects/deflects most spells. Harry practices this technique and eventually uses it to reflect Lockhart’s memory charm. Then he puts Lockhart to sleep with the sleepy knife. (Dumbledore forgives him this technical violation of the rules because it was a non-lethal use in a legitimate emergency.)