He writes on his blog:
Fast App Resume (FAR) is a purely technical setting that controls what the OS should do with existing instances of the application when the user does something which would, traditionally, try and launch a new instance.
Enabling FAR tells the OS that it should load any exiting application instance before starting a new one.
FAR does not define what should happen with the existing instance is NOT universally defined. Each app should do what is best/most appropriate for that app if begin launched via FAR.
The takeaway here is twofold. First of all developers need to think carefully about how their app should react to being opened, when they should return to the last state, and when they should open up and start over afresh.
The second is that Microsoft should really offer some guidance to developers on how apps returning to the foreground should be treated, so every app can act in the same way, and windows Phone retains a high level of integration and familiarity across the board.
you can read all of Matt's thoughts on his blog.