It's actually quite simple. Set it up as above. In the field, press the AF-ON button when you need it to be focusing, and release it when you want it to NOT focus.
For example, if you're shooting baseball from behind the home plate screen - say, the pitcher. You know where he's going to be, and if you're not RIGHT against the screen, you may get some interference. So AF-ON aiming at the pitcher and lock on. Now let go of it. Release the shutter as necessary. Let's say you're shooting a bird in a tree; there are lots of messy branches around. AF-ON to get focus. Unfortunately the AF gets branches instead. Let go of AF-ON, grab the AFS/HSM focusing ring and twist until the bird is in focus. Release the shutter. Different scenario: shooting vintage race cars, panning as they round a corner generally toward you. Press AF-ON as they start to come into range, hold AF-ON, release the shutter several times, release AF-ON.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!