Never saw it called,,, could someone explain this rule and where it is applied ?

Thanks

exempli gratia
 

2 out; R2.

BR singles and is out at 2B in an attempt to stretch.
R2 misses 3B, but touches home plate before BR is called out @ 2B.

Defense appeals R2 @ 3B to cancel run.

Speaking ASA.

R1 & R2 at the corners, 1 out. Fly ball caught by F9 for out #2. Both runners attempt to advance, but R1 left 3B early. R2 falls on the way to 2B and is thrown out, but not before R1 has already crossed the plate. Ruling would be, 3 outs, run scores.

Even though there are already 3 outs, the rule allows the defense to make a legal appeal in an effort to nullify a run. Therefore, if F2 appeals that R1 left early and the umpire agrees, a "fourth out" is ruled in order to take that run off the board.

Hope that helps,