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There is no way to enable backwards compatibility on your brand new Xbox One, despite a message currently being circulated online. No combination of button-presses and menu selections will unlock the functionality, and - in many cases - are more likely to brick your console.
This is reminiscent of old-school Windows PC pranks where people were encouraged to delete their system32 folder, or press Alt+F4 to do just about anything other than quitting a program.
But - just like those were widespread, this new one threatens to do the same. It takes advantage of "less-savvy" gamers - of which there are many - and convinces them to do something complicated enough to seem legitimate.
Simply put, Xbox One owners are asked to load up the developer console through a series of button presses, and then change their sandbox ID to "freezone.reboot". As you might suspect from the second part of that command, this does not enable backwards compatibility. Instead, it prompts the console to start a reboot loop, which makes your shiny new console little more than an expensive brick.
The developer console has been included in the Xbox One firmware primarily for indie developers, making it easier for them to build games for the console. While it can do a bunch of stuff, it cannot overcome the hardware issues that make backwards compatibility unlikely on Xbox One.
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