I think what he's referencing is the old adage, "keep your eye on the ball".
in reality, a hitter's eye is simply unable to follow the complete flight of the ball. but, the more they can follow the ball, the more successful they will be at the plate.
All too many 'hitters' ... *ahem* ... pull off the ball, and don't "keep their head in" to watch the flight of the ball.
I've studied (and coached) a number of hitters. And, the hitters you can work with, and coach, to get to follow the flight of the ball will have a much, MUCH better average contact experience (average, exit velo, etc).
If they don't "fly open"... if they don't get out front of the ball, if they can be disciplined at the plate ... they'll have much better results.