OBR 2.00 · OBR 6.01(h)

Obstruction

Obstruction is the mirror of interference: a fielder without the ball — and not in the act of fielding it — impedes a runner. The umpire's job is to nullify the act, awarding whatever bases the runner would have reached had it not happened.

Key points

Type A (a play is being made on the obstructed runner): ball dead immediately, at least one base beyond the last legally touched.
Type B (no play being made on him): play continues, and the umpire awards bases afterward to cancel the harm.
A fielder actively fielding a batted ball is protected and cannot obstruct; a fielder merely 'about to receive' a throw is not.
The award is judgment: what the runner would have reached clean, with a minimum of one base in Type A.

In a game

Rounding third, the runner trips over the shortstop standing in the basepath watching the play, then is thrown out at home.

Ruling · Obstruction. If the umpire judges he would have scored without the stumble, the run counts — even though he was tagged.

Why it moves the scoreboard

Can award runs and bases for contact nobody saw coming — obstruction calls have ended World Series games.