I actually don't know anyone who ever used this Connector, but here's some Info I found :
Make sure your video card has a so-called "feature connector" on its top edge - cheap video cards often do not have one. This mysterious little connector is not visible from the rear of the computer, only when you open the computer up.
Why do you need one? A video converter card (which you may buy in addition to your VGA card and plug into a nearby slot) enables you to feed images into and out
of your PC from other video sources eg a camera or video recorder.
Many of the popular video converter cards need to plug into the "feature connector" on your VGA card via a cable. Thus, if you don't have a "feature connector" you are immediately kicked out of the multi-media marriage's reception party.
It seems it was used in the past to get a live Display from pure Video Editing Cards, who did not possess an own Display Device.
I reckon the feature Connector allowed those Cards to directly write Video Data into the Card's Framebuffer, thus allowing realtime Video Editing.
I might be in error, but that's how it looks to me..
The VGA Standard Feature Connector appears to be invented by IBM, so a deep search into their Reference Documents should turn up a Document of specifications somewhere.
There are also VESA Standard specifications for Activation and Use of this Connector, since it has been around for a very long time.