It (among other dev stuff) is described in this thread. It seems like something that connects to the 3DS through an external interface, but there aren't many places where it can connect.
There's only two connectors on a 3DS designed to fit a cable of any kind: the charging port and the headphones port. I only see two pins on the charging port, so I don't think it lives a double life as a video out port. The headphones port doesn't seem likely either, for obvious reasons. (Sometimes those ports have another pin in them, but it wouldn't be enough for a digital video signal.)
There's also the possibility that it's wireless, but that doesn't seem very likely. I doubt the IR port would have enough bandwidth, though interestingly one of the pictures of the unit does show something labeled "IrDA". Wi-Fi could be possible, but I doubt it could support both a streaming video and Wi-Fi gameplay. And it would need to support the latter--game developers might want to capture video of online functionality for trailers or playtesting. Besides, if Nintendo had a means of streaming video from a 3DS wirelessly, they probably would have just included it as a feature in the retail devices.
The most likely place, however, would be the game card slot. A device would plug into the game card slot in the 3DS, and a game card would be inserted into a slot in the capture device. This is supported by the fact that the capture device has a label on it that says "FLASH CA" (the rest is obscured by a cable) though it looks like something other than a game card is in there.
If someone outside of Nintendo could figure out how this device works (or if there's a leak, I guess) that would be great for people who want to make YouTube videos or Twitch streams. Right now their only option is to get an expensive aftermarket capture card, the most cost-effective method being to buy a 3DS with one pre-installed.
Of course, there's one more burning question: why doesn't Nintendo just mass-produce this device and sell it? Or just put an HDMI out port on the 3DS? Seriously, I want to know.
There's only two connectors on a 3DS designed to fit a cable of any kind: the charging port and the headphones port. I only see two pins on the charging port, so I don't think it lives a double life as a video out port. The headphones port doesn't seem likely either, for obvious reasons. (Sometimes those ports have another pin in them, but it wouldn't be enough for a digital video signal.)
There's also the possibility that it's wireless, but that doesn't seem very likely. I doubt the IR port would have enough bandwidth, though interestingly one of the pictures of the unit does show something labeled "IrDA". Wi-Fi could be possible, but I doubt it could support both a streaming video and Wi-Fi gameplay. And it would need to support the latter--game developers might want to capture video of online functionality for trailers or playtesting. Besides, if Nintendo had a means of streaming video from a 3DS wirelessly, they probably would have just included it as a feature in the retail devices.
The most likely place, however, would be the game card slot. A device would plug into the game card slot in the 3DS, and a game card would be inserted into a slot in the capture device. This is supported by the fact that the capture device has a label on it that says "FLASH CA" (the rest is obscured by a cable) though it looks like something other than a game card is in there.
If someone outside of Nintendo could figure out how this device works (or if there's a leak, I guess) that would be great for people who want to make YouTube videos or Twitch streams. Right now their only option is to get an expensive aftermarket capture card, the most cost-effective method being to buy a 3DS with one pre-installed.
Of course, there's one more burning question: why doesn't Nintendo just mass-produce this device and sell it? Or just put an HDMI out port on the 3DS? Seriously, I want to know.
