Minecraft wiki entry for Mechanisms, associated text: This is the False3D Monitor, if you place a second row of sticky pistons above the first row you can produce 2x2 pixels. To complete the look of the monitor, you need to place blocks in front of the sticky pistons to determine what color the pixels are, example: wool. This happens because when the sticky piston pushes the "pixel block" and then the shading and the 3D cause the pixel to be visible by the player(s). This can generally be used for any purpose regarding visuals, it can also be used for a mob trap if used properly. And there you have it.