Nearby is the metal armature, and the upright book. 9 leaps off the plank onto the armature, which swings him around to the book. He kicks it and it falls into the pit. The string, tied at one end to the book, has its other end tied to the plank. The weight of the falling book drags the plank into the pit, and the Beast with it. The Beast crashes through a hole in the cellar floor and is presumably impaled by the sharp end of the falling plank. This all reveals that all the things that 9 has built was a cleverly-made trap for the Beast.