Yup. The way they said it was that you put the 360 disc in the Xbox One drive, it reads the disk and it manually downloads the digital version that I assume is the version that makes it work. You just need to have the disk like you do retail Xbox One games.
Yup, think that sounds right. So as far as the average consumer is concerned, they are putting a disc in and playing it - which to them is the same as hardware backwards compatibility. Nice move Microsoft. Nice move...