It's not a disabled/enabled thing - the yes/no shows the current status of the process. Here, Safari webcontent shows it after a while, as does say, Terminal & Finder if no windows are visible for them.
Yes, exactly so.
The greatest benefit to the user comes when every application on their system is doing as little work as possible. Therefore, App Nap is an opt-out feature. The user can opt an application out of App Nap manually with a checkbox in the Finder “Get Info...” pane. Developers can temporarily opt an application out by bracketing user-initiated activities with new NSProcessInfo API. Please consider the power usage of your application before opting out of App Nap.
App Nap reduces power consumption by completely suspending your app’s execution when it meets certain criteria. This ensures that your app does not periodically wake up to do unnecessary work. An app is considered to be a candidate for sleep if:
- It is not visible—if all of an app’s windows are either hidden by other windows or minimized in a hidden dock, and the app is not in the foreground
- It is not audible
- It has not explicitly disabled automatic termination
- It has not taken any power management assertions
When all of these conditions are met, OS X may put the app to sleep. While asleep, the app is placed on a scheduling queue that rarely gets actual time on the CPU.
The app wakes up automatically when the user brings the app to the foreground or when the app receives a Mach message or Apple event.