I hear what lol101 is saying, and that is confirmed on p112 of the good book.....however....
I have found that if you set this feature to on, and using auto focus (spot), the focus grabs first time, every time, no hunting, no checking, it simply locks on.
it could be a fluke (and I will check it again) but that was my experience last week with it.
Its easy enough to test for yourself.
Cheers.