I thought the hologram consisted of 12 projectors, one being from above and one below to set the structure... not a piece of special glass. XD
By tricking people. It's not a hologram at all. It's an updated version of an old stage illusion called Pepper's Ghost. Typically they'll use the same setup as Musion Eyeliner. What they do is have a regular projector overhead pointing down(or a LCD or plasma TV in the floor facing up. Then there's a thin metalized film at a 45 degree angle. The film in nearly invisible but reflects the image. It then appears to be floating in the air on the stage. The thing is though, unlike a real hologram, The image is still 2D. It has no depth. It only appears 3D because it does look like it's floating above the stage and because they use 3D computer software to create the original images. And because people are sitting far enough away at certain angles. On closer inspection it would obvious the image is flat.