Here's a little taste of the article:
"The players weren't exactly sure how the man in white knew what was coming -- maybe, they thought, he was receiving messages via his Bluetooth from an ally elsewhere in the stadium who had binoculars or access to the stadium feed. But they quickly picked up the wavelength of his transmissions: He was raising his arms over his head for curveballs, sliders and changeups. In other words, anything besides fastballs.
-Amy K. Nelson and Peter KeatingA few of the players in the bullpen turned their backs to the field to fixate on the man in white, while others watched the stadium's radar gun. As soon as each pitch was thrown, those watching the man would call out what they thought he was signaling, and those focused on the radar gun would confirm his signal. Sure enough, the man in white was raising his arms above his head before every off-speed pitch and doing nothing when the pitch being called was a fastball."