I was thinking about this again walking to work this morning. There's that scene in Bull Durham where Costner tells the guy what pitch is coming and the guy hits a home run. Intuitively we think, yeah well of course he hit a home run, he knew what pitch was coming. But when you think about it, why exactly did that make it easier?
From the time a 90 mph pitch leaves the pitcher's hand, the hitter has about .4 seconds to try and hit it. Let's take the simplest example of "see ball, hit ball." From the moment the wind up starts, the batter begins picking up subtle cues about what pitch might be coming. Looking for tips, trying to see the grip. When the ball leaves the pitchers hand the hitter looks for speed, trajectory and spin. At some point he will have to decide to swing and also how to swing (where the barrel needs to be delivered). The gathering of data on the pitch while the ball is in flight actually takes up some fraction of the allotted .4 seconds, so the sooner the pitch is recognized, the more time the hitter has to execute the swing.
Now what happens when the hitter correctly guesses what pitch will be thrown rather than just playing see ball, hit ball. My thought is that the swing for the location and the pitch guessed at is already taking shape. The mind is priming the body to execute a particular swing. Next, once the pitch is delivered, the mind is not trying to decipher what pitch is coming, but instead is merely confirming that the pitch coming is what was expected. The confirmation happens much more quickly than recognition from scratch. Time is gained. And because the mind primed for both the pitch recognition and the swing type, more time is gained as the swing sequence also begins earlier. I have to believe this leads to more effective swings. There's time to lock in the exact right swing through small refinements.
The opposite would seem to be true as well. When a hitter guesses wrong he loses time because he's first had to check for confirmation and then when the mind recognizes that it has guessed wrong, it has to scramble to figure out what is coming and try to execute a swing other than the one it had primed for. That's why you get all those funky, awkward swings when a player guesses wrong.