Microsoft also isn't hiding it: "The digital titles that you own and are part of the Back Compat game catalog will automatically show up in the “Ready to Install” section on your Xbox One. For disc-based games that are a part of the Back Compat game catalog, simply insert the disc and the console will begin downloading the game to your hard drive. After the game has finished downloading, you will still need to keep the game disc in the drive to play."
http://www.xbox.com/en-US/xbox-one/backward-compatibility
Sure they might be downloading the 360 version and then doing the emulation but it's much much more likely that they've actually simply ported the game.
It downloads the entire game for each disk game you put in, seems to me that the could've found a more efficient way to actually do that since you know both the 360 and the XboxOne support installing games from disk so they got the ability to create a disk image.
If all they needed to download is an emulator and a compatibility config file that would've been a much smaller download package..
"We have to do packaging and validation work on each title to make it available through Xbox One backward compatibility," explains a Microsoft spokesperson.
Xbox One Backward Compatibility is an Xbox 360 emulator that runs on Xbox One and is used to play Xbox 360 games," says a Microsoft spokesperson in a statement to The Verge.
http://www.theverge.com/2015/6/15/8785955/microsoft-xbox-one...
>Delving deeper, Spencer explained exactly how the emulator packages the Xbox 360 games, and how it compares to Xbox 360's emulation of original Xbox games.
>"You download a kind of manifest of wrapper for the 360 game, so we can say 'hey, this is actually Banjo, or this is Mass Effect. The emulator runs exactly the same for all the games.
">I was around when we did the original Xbox [backwards compatibility] for Xbox 360 where we had a shim for every game and it just didn't scale very well. This is actually the same emulator running for all of the games. Different games do different things, as we're rolling them out we'll say 'oh maybe we have to tweak the emulator.' But in the end, the emulator is emulating the 360, so it's for everybody."
>Asked about whether Microsoft would require permission from game publishers to adjust game code, Spencer clarified it would not be interfering with code.
>"The bits are not touched," he said. "There's some caveats, and as always I like to be as transparent as I can be on this: Kinect games won't work from the 360, because translating between the Kinect sensors is almost impossible."
http://www.gamespot.com/articles/revealed-how-xbox-one-can-p...
I also remember watching a video where they talked about it, it had some more details. I can't remember what it's called though and I couldn't find it with a cursory search.
Behind the scenes, Microsoft has built a full Xbox 360 emulator for its Xbox One console. "Xbox One Backward Compatibility is an Xbox 360 emulator that runs on Xbox One and is used to play Xbox 360 games," says a Microsoft spokesperson in a statement to The Verge.
http://www.theverge.com/2015/6/15/8785955/microsoft-xbox-one...
the package they are sending when you insert the disc is a container that contains the xbox emulator, any media assets, the game and compatibility stuff that is needed.
For example running original Xbox games on Xbox 360 also required a download of a binary which was an emulation / compatibility wrapper around the binary on the DVD that shimmed out and hotpatched code so the games worked properly.