@BarnacleBill is right, magicians dont give away their secrets. but David Copperfield is no magician, he uses camera tricks and special effects, as well as stuges, just what he used to make the statue disapear.
Basically, he puts the camera on a boat, he puts up a curtain, he moves the boat, he takes down the curtain, and shows you an empty bit of water. and a load of paid stuges all back him up by claiming its gone, when really they can all see it just a little to their left.
Some magicians will argue that camera tricks are ok because its just another way to trick people, some will say its a cheat as there is no traditional skill to it.