Apparently by shorting the pins, the video card will report that there is an unknown external display attached.

The DE-9 connector (above) used for CGA, MDA, and Hercules cards, and DE-15 connector (below) used for VGA In 1987, IBM defined a new graphics standard to push the graphics on their PC to levels ...

A feature of a high-resolution graphics card that does not support standard VGA resolution. The VGA card in the computer is cabled to the pass-through circuit on the high-res adapter. When high ...