One of the nicest features in the Windows Phone (WP7 or WP8) is fast app switching (FAS), using which a user can very easily switch between currently running apps and without losing their last running states. So it seems like multitasking experience to the user. But in Windows, the movement you navigate away from the currently running application, its code execution is stopped even though it was running on the asynchronous thread and again started when you launch the application. Hence, though Windows Phone does support background execution of Tasks and the FAS system seems to be multitasking, but in actual it is not a multitasking. It only give multitasking like experience. Because in Windows the application does not continue to execute code while it is not in the foreground. The important thing is to know what happens technically when you navigate away from the app and again launch the app. This post describes how to enable fast app resume for Windows Phone 8 apps and what happens when FAS occurs.