The very last developer preview for Android N was recently released out to the public, making this the fifth developer preview build that Google had planned to push out before sending Android 7.0 Nougat out to the public. Throughout the different builds of the dev preview there have been a handful of changes, improvements, and tweaks to not just the system software itself, but to various apps that are part of the package and installed on Nexus devices. One such application is the Google Camera, and while you can install this on non-Nexus phones, this particular set of tweaks refers to the version which can be found on the Android N Developer Preview 5.
This version of the app should push users on compatible devices up to 4.1, and it includes a feature that allows users to swap between cameras without actually touching the display and interacting with the user interface. For those wondering, it's a gesture, but not like the gestures you might see in launchers such as Nova Prime, rather a gesture you might find on Moto devices. The gesture allows users to twist the phone in order to swap back and forth between both the front and the back cameras, making things a little simpler as you won't have to fiddle with touching the menu buttons. Just a quick flick of the wrist will switch things to front camera if you want to change things up from regular photos to selfies.
Now for users who have a device running on the Android Developer Preview 5, just any sort of twist won't do. You'll need to twist it either on its vertical axis while in portrait mode, or on its horizontal axis while in landscape mode, and by the sounds of it you presumably twist it once to move the camera opposite of the one you're on when you open the app and the viewfinder pulls up, and twist once again to bring it back to the initial camera. As noted this only works in Android Developer Preview 5 or Android 7.0 Nougat, but since there aren't any devices running Android 7.0 Nougat just yet it seems to be limited to those on the final dev preview build of the software.