Fast app switching lets apps to pause and resume themselves quickly and easily change to other apps.An app that is on the screen is "running". An app that is not on screen but is still in memory is "suspended". All apps that are built with iOS 4 SDK support fast app switching if running on iOS 4 and a compatible device. Users can use fast app switching by opening the multitasking bar. The multitasking bar can be displayed by double-tapping the home button.