The MLB pitch timer changed baseball because delay can now change the count. To understand the rule, track who is responsible for being ready, whether runners are on base, and whether the pitcher used a disengagement.
In MLB, the pitch timer uses a 30-second timer between batters, a shorter timer with the bases empty, and a longer timer when runners are on base. The current MLB glossary describes 15 seconds with the bases empty and 18 seconds with runners on base.
The batter cannot wait until the final instant. MLB requires the batter to be in the box and alert to the pitcher by the 8-second mark. If the batter violates that requirement, the penalty is an automatic strike.
The pitcher must begin the motion to deliver before the timer expires. If the pitcher is late, the umpire adds an automatic ball to the count. That can create a walk, so a timer violation can directly put a batter on base.
With runners on base, a pitcher can reset the timer by stepping off or attempting a pickoff, but those disengagements are limited. MLB generally allows two disengagements per plate appearance, with the limit reset if runners advance. A third pickoff attempt must succeed, or the runner is awarded the next base.
The pitcher satisfies the pitch deadline by beginning the legal motion to deliver before zero; the ball does not need to reach the plate before the display expires. The timer begins after the pitcher receives the return throw and play is ready, subject to the operator and umpire procedures. A batter must be in the box and alert with at least eight seconds remaining, which means looking toward the pitcher and being reasonably ready for the delivery. An umpire can pause or reset the timer for injuries, equipment issues, unusual delays, or other circumstances allowed by the rules.
After two disengagements in a plate appearance, the pitcher may still attempt another pickoff. The third action is not automatically a balk or advance, but it must retire a runner or otherwise produce an allowed outcome; an unsuccessful attempt awards runners one base. If any runner advances during the plate appearance, the disengagement count resets. Mound visits, injury timeouts, and an offensive timeout are handled separately rather than automatically consuming one of the pitcher's two ordinary disengagements.
An automatic ball is added to the existing count for a pitcher violation, so a violation on ball three awards first base. An automatic strike is added for a batter violation, so it can become strike three and complete a strikeout. The pitch is not thrown on either call. Timing rules used by college, high-school, youth, and international competitions can have different clock lengths, readiness points, or penalties; the 15-, 18-, and 30-second figures on this page describe current MLB play.