With upconversion, if your monitor is 1024 pixels wide and you upconvert to 2048 pixels then display it on 1024, it looks better.
          Sensible Cinema sets the default default resolution to be 2x your screen resolution, which is better for high end cpu's.
          If you have a low powered system or laptop you may need to change it (drag the slider) lower to avoid it displaying only a black or white screen, as your system can't handle it,
          so adjust accordingly if you have a lesser system.  Also changing this setting can make it use more or less CPU overall.