This is the technique used by the Voodoo cards to put images on the screen of the monitor. There is the "loopback cable" that goes from your existing 2D card to the Voodoo based card, from there your old cable goes to the monitor. For 2D operation you still use your old 2D card the info has to move through the poorly shielded loopback cable to the Voodoo and than to the monitor. This results in quality loss (noise picked up by the loopback cable). When you turn on 3D acceleration the Voodoo card disconnects (using relays or FET-switches) your 2D cards signal and start broadcasting. These switches are again a big source of noise. This noise will result in blurry high resolution images when using your 2D card. It is visible with resolutions of 1024x768 and higher.