I found this article by Canon on using their back auto-focus. Very interesting read, I hope others without Canon's DSLR find the article useful.
If you like to lock focus and re-compose your subjects, you’ll find back-button AF very helpful: Once your focus is set, you can move the camera and take as many shots as you like without AF trying to re-focus on what’s now in the center of your picture.
Photo credits: Aughty Venable.
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