Often when shooting in a studio, you have to step back or angle the camera for a full body shot; and the result often shows unwanted parts of the room beyond the
seamless backdrop. This tutorial shows you how to quickly extend that background in Photoshop!
Why back up 1 pixel? Often if you don't do this it will leave a very thin white line (background color) which you can only see if you are zoomed way in. So backing up the 1 pixel prevents that.