I have finally gotten a crasp on this paticular custom setting, and reading around the net there seems to be quite a bit of confusion about it. While shooting this weekend with AF4 on I managed a shot of a JuviNH that I thinks gives a good example of this function. While tracking this bird I acheived focus lock (AFC) then he headed straight for cover as you can seen focus stayed locked on the bird even though my last shot was covered by reeds between me and my subject. As I have come to understand this function without AF4 set to on auto focus would of lost lock and began hunting again. Here is the shot; If I'm wrong someone straighten me out and if not I hope it helps others. I know it seemed to bring it home to me.