"I learned a lot from last year. I failed a lot and dealt with struggle," Hahn said. "So this offseason, I sat down and looked back at it and figured out where I went wrong and what I needed to change."
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The answer was pretty simple: He was throwing too hard, which resulted in his bread-and-butter sinker staying up in the strike zone and not, well, sinking.
"You don't want to overthrow and throw through your sink. If you have sink, then use it," Hahn said. "Last year, I saw a little bit of a velocity increase, and I tried to run with it. That just wasn't my style of pitching. That's just not me."
A's manager Bob Melvin recognized the version of Hahn that the A's acquired before the 2015 season from the Padres on Monday, when the right-hander threw four scoreless innings of relief against the Reds.
"The ball was down in the zone, a lot of balls beat into the ground. When we've seen him in the past pitch well, that's what he's done," Melvin said. "That's the way we expect Jesse to pitch. Even if he has to back off the throttle a little bit on the fastball velocity, the sink is what made him successful two years ago."
Hahn entered last season coming off an impressive debut season with the A's. He registered a 3.35 ERA over 16 starts and 96 2/3 innings in 2015. Opposing hitters hit just .238 against him.
"I didn't try to be too aggressive. I let my sinker sink, didn't try to overthrow it," Hahn said of his latest spring appearance on Monday. "That's the pitcher I've always been."
Hahn came to camp this season with the goal of making the rotation -- and then staying there all season. He appeared to have an edge for the final spot early on, but with Sonny Gray expected to start the season on the disabled list, Hahn's chances have increased.
"We got a long way to go, but he came into camp with that in mind," Melvin said. "And certainly what he did a couple years ago would suggest he has the ability to pitch well at the big league level and pitch well consistently."