Read original post by Dhananjay Kumar at DotNetSlackers
Windows Phone 7.5 operating system provides improved fast application switching over Windows Phone 7 operating system. Before we go into details of how FAS is better in Mango, let us first understand what is FAS.
Fast Application Switching
Imagine application "A" is running and user launches a new application "B". On pressing hardware back button user can return back to application "A". If application "A" gets activated without "resuming " screen then it can be termed as "Fast Application Switching".