Washington was up, 5-0, and Fister had fallen behind in the count, 1-0, when he fired an 80 mph changeup that looked to be below the strike zone. Diaz initially ruled it a strike with a relatively subdued pump of his fist, and Andrus immediately appeared to express his disagreement with the call.
Diaz also seemed to hear about the call from the Rangers' dugout. Before Fister's next pitch, Diaz raised his hands and emphatically waved his left hand with two fingers up, showing the count had been changed to 2-0.
"I have never seen an umpire do that," veteran play-by-play man Bob Carpenter said on the Nationals' television broadcast.
Andrus fouled off Fister's next pitch, took another ball then hit a ground-ball double to left field.
Adam Berry is a reporter for MLB.com. Follow him on Twitter at @adamdberry. This story was not subject to the approval of Major League Baseball or its clubs.