With Mallex Smith at the plate and Erick Aybar at first, D-backs reliever Tyler Clippard threw a pitch in the dirt in the seventh inning of Friday's 7-2 win over the Braves.
Castillo blocked the pitch and Aybar attempted to advance to second on the play. Castillo got up, and with Smith standing in the left-handed batter's box, Castillo had to throw sidearm around Smith.
"When I pick a ball in the dirt I always look to first base in case the runner gets too far off and I saw him go," Castillo said. "I tried to avoid the hitter. I had no chance, so I just threw the ball where I thought I should throw it.
Ahmed raced to cover second and as he got there, the throw was a bit to the second-base side of the bag and he almost did a split while sliding and reaching for it. In the process, he was practically laying in front of the bag when Aybar slid head first into him.
Ahmed applied the tag as he fell back towards the base.
"He threw sidearm behind the runner and [Ahmed] made a good play and also had to worry about the runner hitting him hard because he was in front of the bag," Greinke said. "So it took some guts to do it too."