CLEVELAND -- It was part of Andrew Miller's routine. Near the end of his spring mound workouts, the big lefty would move into the stretch and fire a handful of pitches with a slide step.
During this particular bullpen session in the spring of 2012, former Red Sox manager Bobby Valentine was keeping a watchful eye on Miller. The lanky left-hander had worked hard on a windup that included a high leg kick, but there was something about the way he fired a few pitches when abandoning that approach.
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"What was that?" Valentine asked Miller. "Why don't you do that every time?"
Miller told the tale on Sunday with a slight grin on his face. He knows Red Sox fans do not look back on the one-season Valentine Era fondly, but said he has to give credit to the former manager. Valentine's enthusiasm about Miller moving to the stretch on a full-time basis helped convince the reliever to go that route.
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Four years later, and now with the Indians, Miller is not only one of the most overpowering relievers in the game, but one of the most dominant in baseball history. There is a growing list of batters who have crumpled to the dirt while trying to make contact with his slider. The 6-foot-7 southpaw can make any hitter look overmatched, and he has given Tribe manager Terry Francona a versatile weapon for high-leverage situations.
Francona will surely employ his weapon plenty in the American League Division Series against Boston, beginning Thursday with Game 1 at 8 p.m. ET on TBS. When Francona managed Boston, he remembers getting Miller in 2011 and thinking that the lefty -- much like Valentine believed -- was capable of unlocking his talent by simplifying things on the mound.
"I didn't know he'd be this good," Francona said with a laugh. "I mean, shoot, I thought that there was a guy in there, like everybody else. I wasn't the only person. Everybody saw it."
Fresh start in Boston
The spotlight had been on Miller since he entered professional baseball.
He was the sixth overall pick in the 2006 Draft by the Tigers, who traded him in December 2007 to the Marlins in the blockbuster deal that brought slugger Miguel Cabrera to Detroit. A project or not, Miller was constantly dealing with the attention that comes with being a high-profile prospect and then a shiny piece in a franchise-altering trade.
When Miller signed a Minor League deal with the Red Sox prior to the 2011 season, it was a welcomed change of scenery.
"I was just trying to find a way to get to the Major Leagues and be a useful pitcher," Miller said. "Boston was a place I could go where I kind of got a chance to shake the whole prospect thing. I had gone from maybe the hype to just trying to figure it out. That had passed."
Miller was still trying to find a home as a starting pitcher. He had worked mostly out of the stretch at the University of North Carolina, but his wildness as a pro (he averaged more than five walks per nine innings through 2010) left him searching for a solution. Pitching coaches had suggested trying to work with a higher leg kick out of the windup, so Miller gave it a shot.
Gif: Andrew miller strikes out player
Francona kept preaching to Miller that spring to focus on his work, simplify all the moving parts in his throwing motion and get to the big leagues when it made the most sense. Miller wound up making 17 appearances (12 starts) for the Red Sox in 2011, but the results were mixed.
"I was trying to convince him that Opening Day wasn't the finishing line," Francona said. "When you're that age and you're trying to make a team, I get it. But, I was trying to convince him that, 'Hey, if you can get yourself simplified, man, there is no ceiling.'"

Miller's tranformation
Even after the conversation with Valentine, Miller was trying to win a spot in Boston's rotation for the 2012 season. That changed when the lefty felt stiffness in his throwing elbow early in the preseason.
That injury may have been a blessing in disguise, though. When Miller made his return to the Majors in May, he did so exclusively as a reliever. He committed himself to pitching only in the stretch and shifting to the slide-step approach that felt so comfortable. There was still plenty of work to do, and work Miller did. He made 53 appearances, seeing his velocity spike and his walk rate drop.
"In 2012, I doubt too many people warmed up more often than I did," Miller said. "But, I think it was great for me. I got off the mound 100-something times over the course of six months. I didn't get into the game every time, but I had a chance to kind of work on my craft."
That was the beginning of Miller's road to dominance.
What is on display now is a pitcher in complete control of his game. In 70 games this year, Miller piled up 123 strikeouts with only nine walks in 74 1/3 innings, fashioning a 1.45 ERA between stints with the Yankees and Indians. His 13.7 strikeout-to-walk ratio ranks third on the all-time single-season list (min. 70 innings). Miller is the only pitcher in MLB history with 120-plus strikeouts and fewer than 10 walks in a season.
Gif: Miller strikes out KC player
"He's done some things to maybe harness his stuff," Red Sox manager John Farrell said. "You look at the strike-throwing. He's obviously physically responded well to the short stints, more frequent. But, he went to the no-leg-kick -- you're talking about long levers, long legs and arms -- to really repeat the delivery, and he's done a great job of that."
Dating back to the 2012 season, Miller has turned in a 2.14 ERA with 425 strikeouts in 269 1/3 innings. Over the past three years, the lefty has been even better, posting a 1.82 ERA with 326 strikeouts in 198 1/3 innings. Over the past two years, Miller has thrown his slider more often than his four-seam fastball, and his velocity has held relatively steady.
The Indians acquired Miller on July 31, sending a hefty package of prospects -- headlined by outfielder Clint Frazier -- to the Yankees. With Cleveland, Miller has appeared in every inning from the sixth on, picking up nine holds, four wins and three saves, logging eight multi-inning efforts.

The Red Sox helped turn Miller into what he is today, and now they have to face him in October.
"We know that Andrew Miller is going to be in there," Farrell said. "He's one of the top three or four relievers in all the game."