"The [Yankees] first-base coach could deduce when Price was throwing a changeup and would signal the hitter"

Yahoo:

Unbeknownst to the Red Sox, the Yankees believed Price was tipping his changeup. Only it wasn’t visible to the hitter. The first-base coach could deduce when Price was throwing a changeup and would signal the hitter. With his cut fastball becoming a pitch he used less and less, taking the changeup out of the equation made hitters’ jobs even easier. The Yankees jumped him.

Houston also knew about Price’s tipping and did the same in Game 2 of the ALCS. Over the four days between then and Price’s Game 5 start, the Red Sox cracked the tell. They also eliminated his cutter almost entirely, making the changeup even more important. Price threw a career-high 39 changeups that night. The Astros swung and missed a dozen times after doing so once in Game 2.

His wasn’t Boston’s only on-the-fly fix. As the postseason wore on, the Red Sox feared runners on second base were seeing the unique grip Kimbrel and others use on their curveballs. With a so-called knuckle curve, pitchers bend their index finger and put pressure on the fingertip, exposing the knuckle. Upon seeing the grip from behind, the runner at second can relay the pitch type home – a disaster for Kimbrel, who throws only a fastball and curveball.