The troll roared again and twisted its shoulders back and forth, trying to fling Harry off itself. Harry was barely holding on by one hand around its neck. He felt wildly with his other hand and feet, trying to find some purchase, and his left foot landed on top of the knives already in the troll's back. He pushed up—the troll let out an earsplitting bellow and actually dropped its club—and managed to lift himself onto the troll's shoulders.