OBR 5.09(b)(5) · OBR 5.09(c)

Tagging up

On a caught fly ball, a runner must retouch his base before advancing. The clock starts at the fielder's first touch of the ball — not the completion of the catch — so runners can leave the moment the ball hits the glove.

Key points

First touch, not secure possession: a bobbled fly can be tagged up on early, which changes the risk math on deep balls.
Leave before first touch and the defense can appeal — the runner is out if the appeal beats him back.
If the fly is dropped, no tag is needed; normal force rules apply.
An appeal at the base is not a force out — a run that scored before a successful third-out tag-up appeal still counts.

In a game

Sac fly to the warning track, runner on third scores easily. Defense throws to third claiming he left early.

Ruling · If the appeal is upheld for the third out, the run is wiped off — tag-up appeals are treated as force plays for scoring purposes.

Why it moves the scoreboard

Decides sacrifice flies and can erase runs after the fact on appeal — the timing of the runner's leave against the first touch is the whole play.