Short video showing how to set up your Nikon D3100 for back button focusing.
Ive had many requests on how to do this, and its really very simple.
BBF is great for sports and action photographers,it allows you to keep focusing whilst shooting, seperates exposure and focus, VR and focus and many other benefits.
Its not for everyone, but give it a try.
To change back to the normal method, just go back into menu and the buttons settings, and change AE-L/AF-L back to that setting.