Chen likes to keep extra pieces of wood or stone handy while he's making the bookends to shim up the mold, in case he wants to alter the angle of the cement (and thus the shape of the final object) immediately after it's poured in. In this case, he lifted the back legs up a bit to make the bookend a little wider at the base. It takes 15-20 minutes for everything to dry, and at that point the bookend should slide right out of the mold.