He is not really mentally picking the string. It is way to fast. It's like setting-up a vibration in his arm, wrist, hand and finger. Here are a couple of things to try to see what I am trying to get across.
1. Hold out you arm and hand (palm facing down) straight and level. Think about 1st moving your thumb side of your hand down (while doing it), then rock it back, rocking your pinky side down. If you think of each move before you do it (which is usually the way one has to do it) you will not be able to go fast at all. Now try to think of setting your arm, wrist and hand into a vibration. It's kind of what you would do to shake water off your hand. If you look at the video again, you can see his arm, wrist and hand in the side to side rotation type vibration motion.
2. Hold your hand out again, palm down, drop your ring finger straight down. Now try the string picking motion with the ring finger only. If you are able to do it you're probably only able to do it slow. Now, do the same thing with your forefinger. Probably a little faster, but "no cigar". Now try it with your middle finger. It should (most cases) go really fast compared to the other two. It is easy to feel a natural vibration movement taking over.
Now go back to lower string (#1), put the tip of your thumb nail and forefinger nail together on either the top or bottom of the string. Try to get your arm, wrist, hand moving up and down in the vibrating motion. Practice it a couple of thousand times and you'll have it! (May have taken me a couple of extra thousand!)
Confused? Yes?