The technique is called Back Button Autofocus and it can really change the way you use your camera. Rather than autofocusing with your shutter release, you move the autofocus function exclusively to a button on the back of the camera. When you first hear about this technique, it’s natural to greet the idea with a bit of uncertainty, but once you get used to focusing with this method, you may never go back.
Autofocusing usually involves holding the shutter release halfway down until the image is focused. However, with a new technique known as Back Button Autofocus you can make the task of focusing your images a whole lot easier. Read this article for more details.