I can't say that I am 100% sure but I believe that is when a pop up is allowed to drop between two or more fielders. The name would come from the 1912 World Series when the New York Giants' Fred Merkle and Chief Meyers allowed the Tris Speaker's foul pop up to drop harmlessly to the ground in the 10th inning of game 7. Speaker then got a base hit that scored the tying run, which was followed by a sacrifice fly that won the game and the Series for the Red Sox.