This is only used if border blanking is off and these values are larger than zero. Each pixel line will be duplicated, making it twice as high as it otherwise would be. Also halves the vertical frequency . DPMS DPMS stands for Display Power Management Signalling. The most common monitor technology today is based on a so called cathode ray tube.
Magnets control a beam of electrons that sweeps over the display.
This beam moves from the left to the right, from the top to the bottom. When the electrons in this beam hit a flourecent layer inside of the monitor glass, that part glows for a short while. By making sure that each part of the screen is updated often enough, you get what appears to be a stable picture. There are other display technologies available, but for now, they all accept the same kind of signals from the computer. So you could say they look like a cathode ray tube to the computer.