Google hasn't said anything about the feature-- XDA just happened to discover the code in a recent build of Android 7.1. Essentially, it listens for back button presses, and if enough of them happen (four to be exact) in rapid succession (with less than a third of a second delay) then Android will override the app and bring back the home screen. This could apply to apps that just freeze, but also to rogue software that tries to intercept any and all actions, like malware or adware.

It's a smart idea, because what's the first thing you do when you can't make an app go away? Frantically pressing the back key is probably the first thing, so that will kill the app and allow you to uninstall or disable it until you figure out the problem.

You'll have to enable the feature to get it to work, apparently. Google seems to be rolling it out on a limited basis, and may in fact just be testing it, so it may be some time before it ends up on your device.