He is a rare breed.

Though Aaron Cook is typically viewed as a conventional pitcher, in some respects, he is an oddity on par with the likes of knuckleballer Tim Wakefield. Cook features a two-seam fastball that does not merely sink but instead seems to run off a cliff as it approaches home plate. It is a weapon that, when effective, can succeed without any complement.

Many starting pitchers feature a sinking two-seam fastball as one option in a diverse repertoire of three or more offerings. But there is a special class of hurlers that not merely throws the pitch but lives by it, with Cook joining a select group that includes perhaps Derek Lowe and Jake Westbrook who have tethered their performances so dramatically to a single pitch.

Cook -- who signed a minor league deal with the Red Sox this offseason, was called up on Thursday and will make his first start for his new club at Fenway Park on Saturday -- recalls one big league start in which he threw nothing but his signature pitch.

“I don’t remember who it was against, I threw nothing but sinkers the whole game,” Cook said with a chuckle. “Didn’t throw any off-speed, didn’t throw any curveballs. Threw it to both sides of the plate, in and out, up and down, and had a fun day.

“I might have gotten a no-decision,” he said. “It was fun, it was definitely fun.  After the game, I had some of the guys come up and ask, ‘Did you throw any breaking balls today?’ I said, ‘No, all sinkers.’ They just started laughing. But it’s something that, when it’s working, I can throw it to both sides of the plate and there’s two different pitches, take a little off, throw it up in the zone, then you change the eye level of the batter and whether they’re looking in or out. That’s what keeps them off-balance, more than the spin or the break.”

That was an extreme outcome, but it is not unusual for Cook to throw his sinker three times out of every four pitches -- sometimes more -- in order to send a ball darting below the barrel of the bat while an opponent swings, resulting in a ball that gets pounded into the dirt.

The pitch is a freakish offering that turns baseball logic on its head. From the day they pick up a baseball, pitchers intuitively know that the swing-and-miss is the most powerful outcome in baseball.

Yet for sinkerballers like Cook, the object is different. He had to learn not just how to throw the pitch but also how to embrace a different goal with it, chiefly, bad contact.

Ordinarily, Cook’s performance in Triple-A would raise red flags. The 33-year-old struck out just 13 batters in 33 1/3 innings, and walked nearly as many.

Yet in his case, the low strikeout rate actually reflected significant success, since it was a byproduct of the fact that his sinker induced contact early in counts, and so he rarely was even in position to throw a pitch by a batter with two strikes. He dominated Triple-A opponents while with Pawtucket, working to a 1.89 ERA in five starts that convinced the Sox to bring him up to the majors.

In a past life, Cook was like 90 percent of his peers. A second round pick by the Rockies in 1997, he had the ability to throw his four-seam fastball hard, dialing it up into the mid-90s, but Rockies coaches noted that the offering had little movement -- a recipe for struggles in the highest levels of the game.

“He had great arm strength and very high velocity fastball, definitely a plus fastball. It was relatively straight but he could command it because he it was the only pitch that he had,” recalled Bryn Smith, then a Rockies minor league pitching coach. “As he started going to the higher levels, we saw that straight is not always good and it can get hit pretty good. So we just started experimenting.”

And so, while working out in Denver in January 2001, Cook received a career-changing instruction session from Smith, then a Rockies minor league pitching coach.

“He said, ‘Hey, try throwing the ball like this and throw it, see what happens,’” recalled Cook. “I threw it and it went down. He said, ‘OK, do it again.’ I did the same thing. It was just a matter of figuring out where I needed to throw it to get it in the strike zone.”

At first, a conventional grip of the fingers on both seams of the baseball at an angle didn’t take, perhaps because Cook threw too hard to generate sink. And so, a tweak was made. Cook ran a finger across just one seam of the baseball.

Eureka.

“He got a hold of it really quick,” said Smith. “Once we ended up leaving spring training and taking it into games, the more success he had, the more he liked it and it opened up a whole new can of worms. … He kind of fell in love with it. The rest is kind of history.”

It wasn’t quite that immediate, but in some ways, it seemed close. Cook spent the next few weeks prior to spring training honing his new weapon. That season, he was assigned to Salem of the High-A Carolina League (then a Rockies affiliate, several years before it became a Red Sox franchise), where he was paired with Bob McClure, then a Rockies minor league pitching coach who now oversees the Red Sox’ big league pitching staff.

Cook had developed a weapon but still needed to work to master it both physically and philosophically, like a young Luke Skywalker who required a guru to show him how to employ The Force. McClure was his Yoda.

“One thing I give Bob McClure a lot of credit for is teaching me how to pitch to contact and trusting that I can get guys out with groundballs,” said Cook. “Swings and misses, for me, are more of a timely thing – certain situations with guys on, less than two outs, less than one out, that’s the time I really try to go for the strikeouts. But other than that, I really try to make guys mis-hit the ball, hit pitches that I’m trying to make.

“I think it’s something that took me a few years in the minor leagues. Working with Bob McClure on it, and realizing what kind of a weapon it was, I went from throwing 110, 115 pitches a game down to throwing 80 pitches a game.”

Cook enjoyed a solid performance with McClure in Salem in 2001, going 11-11 with a 3.08 ERA, but it wasn’t until the following year that he shifted into a higher gear. Reunited with Smith, who was the Rockies’ pitching coach in Double-A Carolina, Cook’s sinker became an overpowering pitch. He went 7-2 with a 1.42 ERA, achieving success that surpassed any in his career despite the fact that he was striking out fewer batters than ever.

“It just took off,” said Smith. “He got to the point where they really didn’t have a chance. For him to dominate like that, that quickly, it just took off and I just got out of the way.

“His is a power sinker,” Smith went on to explain. “It wasn’t like he was throwing an 82, 83, 84 mph sinker. This thing was coming out of his hand at 92, 91, 93. I can only imagine that’s even harder to hit. The movement was fun, but I was amazed with the velocity he was able to get while still getting the ball to move like that. I always tell pitchers, if you throw your sinker too hard, it ends up staying straight and it gets hit. There’s a fine line there. With his, they just couldn’t hit it.”

The pitch proved an express pass to the majors, with Cook flying through Double-A and Triple-A before reaching the majors by August of that year. He soon became a mainstay of the Rockies’ pitching staff, the leader of a group in the middle of last decade who proved that residence in the mile-high altitude of Coors Field was not a guarantee of failure.

Pitchers who could keep the ball on the ground could succeed even in Denver, and Cook became the foremost example of that en route to status as the franchise’s all-time leader in wins, going 72-68 with a 4.53 ERA over his 10 years in Colorado. Cook enjoyed particular dominance from 2004-09, when he was 57-43 with a 4.09 ERA despite striking out just 3.8 batters per nine innings, the lowest strikeout rate in the National League during that span.

The pitch allowed Cook to enjoy performances unlike virtually any other pitcher. Since 2000, there have been just five nine-inning complete games in which a pitcher threw 80 or fewer pitchers. Cook is the only pitcher with two of those to his credit, a 74-pitch effort in 2007 and a 79-pitch shutout in 2008.

Smith left the pro coaching ranks after 2002, but he continued to follow Cook’s career from afar and was amazed by what he saw from his former protégée as his sinker evolved into a pitch like few others.

“Once I got out of the game and got to see him on TV, I almost started chuckling, ‘You guys don’t even have a chance,’” said Smith. “As a pitcher, it’s fun to know if you can throw one pitch, not have to think too much and guys are getting themselves out, man, that makes it so much easier and so much better.”

Cook, who was named an All-Star in 2008, saw his success falter in the last two years while dealing with a host of freak injuries, including recovery from a broken leg and a broken finger suffered last year in spring training that left him without feel for his signature offering all season. This year, he has returned to full health, and he’s seen the life return to his sinker while seeing his primary secondary pitch -- a slider -- round into a sharp secondary option.

But while the slider is a useful complement, it is the sinker that will determine whether Cook succeeds in Boston. On that front, the right-hander believes, he is better positioned for success than he has been in years, his sinker once again plummeting as if sucked into a vortex as it approaches the plate. 

“Over the last few years, it’s head and shoulders above where it had been and I couldn’t be more happy,” he said. “I felt like signing with Boston was a clean slate, a new start and I still have the rest of my career in front of me, however long that is.”