One of the coolest features in the first two Android N previews was the fast app switching feature of the overview button. It was essentially like alt-tab on Windows, and it made it a breeze to cycle through recent apps. In the new developer preview, Google appears to have drastically scaled back this feature. You can only toggle between two apps now.

In DP2, you can still double tap the overview button to jump to the last app you used. This is actually the default behavior every time you open overview. Even if you wait and scroll through the list before tapping the button again, Android still switches to the previous app, not the one you just left. I suppose that makes sense (why are you hitting overview otherwise?), but it's not what Android does right now.

Missing in the new build is the ability to just tap overview continuously and cycle through all your open apps. There was also a timer in the last build that would automatically select an app after you stopped on it for a second. Admittedly, this feature was a bit complex and I could see people getting confused. It may also have just been buggy. Perhaps it will come back later, or this might be the last we see of it.