The magician tricks the audience into looking the wrong way. For example, he puts a frame of some kind in front of the object (eg the Eiffel Tower), so the audience can see the Tower through the frame. The magician then switches off the lights or diverts the audience's attention, swivels the frame slightly, and places something else between the audience and the Tower - in practice, very little lateral movement is required if the object is some distance away. The audience don't notice the swivelling of the frame or the movement of the curtain, and the Eiffel Tower or whatever sieems to have disappeared. (You can try it yourself if you have a window with small panes of glass. Look through one pane and you see, for example, a tree. Swivel your eyes slightly so that you're looking through the adjacent pane, and the tree has gone.)