Posterior.
OBR 5.05(a)(2) · OBR 5.09(a)(2)-(3)

Dropped third strike

If the catcher does not catch strike three cleanly, the batter can run to first — but only when first base is unoccupied or there are two outs. Otherwise he is simply out, which stops defenses from dropping the ball on purpose to start a cheap double play.

Key points
  • A pitch that bounces before the catcher gloves it is not a catch, even if it never touches the ground again.
  • First base occupied with fewer than two outs: batter is out regardless, and runners advance at their own risk.
  • Two outs: batter may always run, and every force is live — this is how four-strikeout innings happen.
  • The batter is out once tagged, forced, or — in most parks — once he leaves the dirt circle around home plate.
In a game

Bases loaded, two outs, strike three gets away. The catcher jogs to tag the batter instead of stepping on home.

Ruling · Every runner is forced, including the runner from third — a step on home plate ends the inning with no run.

Why it moves the scoreboard

Extends innings that should be over and creates force plays from nothing — a live dropped third strike with two outs puts every runner in motion.

Related rules