To enable the Fast App Resume, the developer needs to modify the app’s WMAppManifest.xml file and set the ActivationPolicy value to “Resume” in the DefaultTask element.

<Tasks>

<DefaultTask Name="_default" NavigationPage="MainPage.xaml" ActivationPolicy="Resume" />

</Tasks>

The ActivationPolicy takes the following values

– Resume (Fast App Resume – FAS)

– Replace – Default Value

Link: http://mobileosgeek.com/how-to-enable-fast-app-resume-in-windows-phone-8-app/