They show a nice little wooden box that has plexiglass windows at the top and bottom. The box contains three large cubes (approx. 20x20mm).
 You point the box from above and below, then slide it open. Remove one of the cubes and put it in your pocket. Now you can show the box to the viewer from either side. If you shake the wooden box now, the missing cube will appear. Now you can shake the box again and the cube will disappear and reappear in your pocket!
 No manipulation is required for this routine. Nothing is reloaded or secretly added. The effect is a miracle of self-work. A real gem!