There have been a few discussions around the net concerning how MS has accomplished their BC for 360 games.  Not only is the X1 running 360 games but it is also running the 360 OS as well.  If we think about how the X1 works, there are 3 virtual machines.  One is the main VM that host the games and App VM.  It appears with the 360 BC that MS can either change, swap or add another VM which they are doing with the 360 games.  With this particular tech, what is stopping MS for adding, changing or swapping a PC VM.  

When Windows 10 release, the X1 will be updated with its kernal.  MS should be able to run a WIndows 10 PC VM just like they run the other OS and thus giving them access to PC games on the X1.  At least that is my theory but the question is if possible will MS go down that route.  One benefit is that for indie developers who are already making PC games, no need to developer for the X1 and the X1 gets the benefit of a huge library of games already running on the PC.

And how exactly does that work ?

The overwhelming majority (almost all) games are designed in their own isolated application. They were also designed for PC specs and not for the XB1.

Also if you allow random PC games the ability to install themself on the XB1 -- you are opening up access to allow anyting on the system. Next thing you know, XB1 will need antivirus!

It would work the exact same way as running a Windows VM on a Mac or PC.  You run the VM and execute the application within the VM.  All of the overheaded needed to run a Windows PC are not needed just ability to execute a game.

Also just like the 360 BC, it would be on the approval of the developer and MS.

That would mean Steam on XBO :-o (which would barely benefit MS, in fact it probably siphon 3rd party game sales from XBO to the PC Xbox).

I doubt MS would do it.

True, the possibiliy of MS doing it this way is probably small but doing it through the Windows Store is probably the way its going to go down.

MS has been promising that since the original Xbox.

It isn't going to happen. It would mean days at best until a homebrew if they do, not to mention intra-brand competition between X1/PC games, and said PC games would stutter horribly as they try to page graphics data into the slow-ass DDR3 DRAM.

Sony has slighlt better hardware to pull it, but they aren't going to do it either.

Why not just stream PC games to Xbone?