The Pitch Clock
Fifteen seconds with the bases empty, eighteen with runners on, and it took roughly half an hour out of every game.
What it actually is. A pitcher must begin his delivery within 15 seconds with the bases empty and 18 with runners on, timed from when he receives the ball. The hitter must be in the box and alert with 8 seconds left. A pitcher violating it is charged a ball; a hitter violating it is charged a strike. Pitchers are also limited to two disengagements per plate appearance, meaning step offs and pickoff attempts combined.
What somebody who knows the game gets out of it. The clock is the most successful rule change in modern baseball and its second order effects were larger than its first. Game time fell by about half an hour immediately, which was the goal, but the disengagement limit is what changed the sport: a pitcher who has used both step offs can no longer control a runner, so leads grew and stolen base attempts and success rates jumped to levels not seen in decades. One rule aimed at pace revived a whole part of the game.
The numbers worth knowing. 15 and 18 seconds, 8 second hitter deadline, two disengagements. Average game time fell from about 3 hours 4 minutes to around 2 hours 40. Stolen base success rates rose above 80 percent.
What people get wrong about it. The clock is not paused for a catcher's sign sequence, which is a large part of why PitchCom became universal: an electronic call is faster than a set of fingers, and under a 15 second clock that difference is real.
At Little League and travel ball. Youth and travel tournaments use time limits on whole games rather than pitch clocks, which produces a different kind of pace pressure. The relevant youth rule is usually no new inning after a set time, which is why a team ahead late has an incentive to slow down and why umpires enforce pace at the end of games.
Batter's Eye: look up anything in baseball