You hook the hdmi out from your 360 into the hdmi in of the Xbox One. In theory, that means you can jump back and forth between the X1 and 360. It also means that you can use the 360 while still having quick access to X1 features such as snapping another app, or just switching to another app/game on the X1.
In practice, some people have used it and found it useful, others have noticed the lag during gameplay, making it not an option at all. Its easy enough to try yourself if you want to mess around with it.
Thanks, don't really see a need for it. I sold my Xbox 360. If only Xbox One could see Plex and let me watch movies off my desktop.