The channel's main screen is basically a list of video clips. When you select a clip, the interface jumps directly into a video window, stylized depending on if the footage is related to the Wii or to the DS. The footage is played back in streaming form, so there's no need to wait for a download.
More than anything at the new channel, the service likely to cause the greatest stir is the DS demo download channel. That's right, actual DS game demos, downloaded to your DS, via your Wii. The process is simple enough -- click on the demo, wait for it to download to the Wii, then transfer it over to your DS using the DS's game download option.
Pretty cool eh? Apparently the video quality isn't great, but who cares.. we're more interested in the DS demos. Sadly we don't know when this channel will be rolled out in the US or Europe, we imagine it'll be early next year though..
If you know Japanese, check out Iwata demoing the channel at this years Nintendo Show 2007.