I know, but if it works how I my old capture card did - it shows the screen on both the external display and the computer that's doing the capturing, in a window that can potentially be maximised.
It was really nice back when i used to do it as i could run consoles in a window and carry on doing "important" stuff still. 🙂 I have no idea if the elgato HD thingy works the same way though.
edit: "The image in the Elgato Game Capture HD software on your Mac or PC will be delayed from live by at least a second, so it will not be possible to play the game while viewing it in the Elgato Game Capture HD window. There is no way to turn that delay off." - So.. No it's nowhere near usable. That's a shame. :/ I wonder if it would be faster with SSDs or if the bottleneck is the USB. (my old capture card was PCI)
I remember people being pretty surprised when they added video-in on Displayport iMacs in the first. That was a great feature. Sad to see its not in the latest models.