If your editing DV in FCP (not sure about Avid) and your using a deck or video camera, the feed is sent back out over the firewire cable to said deck or camera, and you hook up your NTSC monitor to the deck or camera. Most of the new Macs come w/ VGA and ADC connector, although I think some that had a certain ATI Video card had S-Video. I'm pretty sure these are only for extending your desktop, not just the editing view.