MLB ABS 101: A Visual Guide to the New Automated Ball-Strike Challenge System.
When is the best time to challenge a ball/strike? Also, what Milwaukee was doing during spring training by putting up a card on the dugout fence earned them a slap from the league office. The rule states that only a player can challenge a call, but MLB viewed the cards that Milwaukee was using as instructions from the coaching staff. Of course, it wasn't a specific instruction to challenge the latest call, but rather it was just letting the batter know that it was a good time to challenge if the batter thought it should be done.
Lindsey with Close Call Sports gives a real quick introduction to the new ABS system.