Question

do you speak baseball?
if you watch a professional baseball game attentively, you will see a subtle game within the game: a sequence of wordless signals passing among members of a team. these seemingly arbitrary gestures—a player scratching his nose and then tapping his right forearm, a coach tugging his cap three times in succession—are actually a secret, closely guarded code to communicate strategy within the team. an adjustment of a cap might be an instruction for a batter to bunt the ball; a hand in the back pocket could indicate that a player should steal a base. as you might expect, opposing teams expend enormous effort to crack each others codes. for this reason, teams not only change their codes often but also intersperse meaningless decoy signals with the real ones, just to confuse the opposing team.
what is the main, or central, idea of the passage?
baseball players and coaches spend quite a bit of time trying to crack other teams codes.
baseball players and coaches use secret signals to communicate strategy to each other.
to maintain secrecy, baseball players and coaches must adapt the codes that they use.

Explanation:

The passage focuses on how baseball players and coaches use secret word - less signals to communicate strategy. While it mentions code - cracking and code - adapting, these are secondary to the main idea of the use of secret signals.

Answer:

Baseball players and coaches use secret signals to communicate strategy to each other.