Spoiler alert: it won't enable backwards compatibility, instead it'll brick your console (specifically, by making it reboot endlessly). It's absolutely marvellous that Microsoft left such a feature in and not included a failsafe such as an automatic boot to safe mode option.
This is going to be a bad weekend to be a retailer with a demo station.
(Apparently if you have a PC and the actual dev software it's possible to fix, but if you're a Gamestop manager that isn't going to be the case. I imagine Microsoft will be shipping a recovery USB to replicate that process next week to retail partners. Because there'll be hell to pay if there isn't.)
No, it's something they lop out of the retail release. Then again, I suppose it's a tie-in for the whole "every Xbone is a dev kit" thing, although they really should make it harder to unlock than "press all of the shoulder buttons in the settings menu".