The magician shows a key that is strung on a cord and worn around the neck. The key and cord are removed from around the magician's neck and he begins wrapping the cord around his hand. As the key is dangling on the cord, the magician magically removes it with a little tug.
The cord is then readjusted and as a loop of the cord is whipped against the magician's palm, the key magically becomes attached to the cord again.
Nice visual magic. Everything you need to perform this miracle is supplied with illustrated instructions.