Excuse me for being naive, but how is this even possible? The time elapsed from the moment a catcher calls a pitch to when the pitchers throws is what, 5 seconds at most. In that amount of time someone would have to turn their attention away from the pitcher and to whatever/whoever is relaying the information. I've never seen a Red Sox game at Fenway (and I've seen many on digital cable) where every batter glances away from the pitcher before he throws. I'd be interested to find out exactly how the 51 Giants told the batters what pitch was upcoming.
Only way I could possibly think of would be if players had tiny headphones in their helmets. But even thats unlikely with an umpire and catcher next to you.