How was Minor League Baseball pitcher Hayden Deal able to throw a two-pitch three-out inning in 2018?

In the tenth inning, the batting team starts with a runner on second base. A first-ball double-play, followed by an out next ball was all it took for this rare event.

With the game tied heading into extra innings, fifth-ranked Pirates prospect Oneil Cruz -- the last batter retired in the ninth -- started at second base in the 10th under the new rules adopted throughout the Minors this season. Deal (7-1), in his third full inning in relief of No. 26 Braves prospect Jasseel De La Cruz, faced Bucs No. 10 prospect Calvin Mitchell. With a chance for the Power to take the lead, Deal expected them to attempt a sacrifice bunt. But the lefty-swinging Mitchell got the green light and pulled an outside pitch to the right side, right to a perfectly positioned second baseman Derian Cruz, who ran to his right and stepped on the bag to nab Cruz -- who was running on contact -- for an unassisted double play. On Deal's next pitch, Deon Stafford lined to third baseman Marcos Almonte to end the inning in the blink of an eye.

https://www.milb.com/news/rome-braves-reliever-hayden-deal-completes-two-pitch-inning-289791644

Minor League Baseball pitcher Hayden Deal threw a two-pitch, three-out inning on August 10, 2018, by taking advantage of the extra-inning rule that places a automatic runner on second base to start the frame. Batter Calvin Mitchell swung at Deal's first pitch and hit a sharp line drive straight to second baseman Derian Cruz. Cruz caught the ball for the first out and immediately stepped on second base to double-off Oneil Cruz, who had strayed too far off the bag, completing an unassisted double play. The next batter, Deon Stafford, swung at Deal's very next pitch and lined out directly to third baseman Marcos Almonte to end the inning. Because the "ghost runner" rule was already active in the Minor Leagues at the time (prior to its adaptation by Major League Baseball), it allowed Deal to log the first known two-pitch, three-out half-inning in professional baseball history.