The magician shows a chain to be solid and strong. The chain is looped through a ring at one end and the loop placed around the magician's wrist. Both wrist are placed side-by-side and the spectator encircles both wrists with the remaining length of chain. A lock is then used by the spectator to secure the loose end to the chain. The spectator holds the key. In an instant and in full view the magician gives a twist and a shake of the wrists and is free!
Easy to do, but very convincing effect.