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CLEVELAND — Josh Hader's fastball is the most dominating pitch of its type in recent baseball history. And it's a complete mystery.

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• This pitcher demonstrates a higher arm slot that would be around 1:00 on the clock face (if standing behind the pitcher looking towards the catcher). Notice the pitcher's shoulder tilt which is created by the orientation of his torso. This is what creates the higher arm slot. Now look at the tilt of this pitcher's shoulders.
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Pitchers with lower arm slots typically release a ball with a more vertical axis that creates more side spin, like a sweeping slider from a lower arm slot. That's in contrast to over-the-top. VIDEO #8 - Sidearm/Submarine Pitching Philosophy; VIDEO #9 - Finding your arm slot; VIDEO #10 - What do scouts look for in a Sidearm Pitcher? VIDEO #11 - Pitching Mechanics: The Set Up; VIDEO #12 - Pitching Mechanics: Linear vs Rotational; VIDEO #13 - Body Tilt/Arm Angle; VIDEO #14 - The Follow Through; VIDEO #15 - Best MLB Pitchers To Watch.

The Milwaukee Brewers reliever has struck out an absurd 50 percent of batters faced this season.Through Sunday's games.

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But what's perplexing about Hader's whiff rate is that hitters know what's coming: He is going to throw his four-seam fastball. Hader turns to his signature pitch on 88.6 percent of his throws, a greater frequency than all but two MLB pitchers to have thrown at least 20 innings this year. While the pitch's velocity (95.9 mph) is above average, it ranks just 66th among fastballs. By comparison, Chapman's fastball averages 98.2 mph, which is sixth-best in the league.

Hader also owns a below-average total spin rate, as calculated by Statcast's TrackMan Doppler radar component. The average spin rate for a four-seam fastball this year is 2,284 revolutions per minute, while Hader's is a rate of 2,154 rpms. Moreover, fastballs — even mid-90 mph iterations — are generally pitches that produce some of the lowest swing-and-miss rates in baseball.

Yet batters are whiffing on 44 percent of their swings against Hader's fastball, the top mark in the majors.Among pitchers who have thrown at least 50 four-seam fastballs, according to Baseball Prospectus.

Since 2008, when pitch-tracking systems were up and running in all major league parks, Hader owns the greatest career swing-and-miss rate of any pitcher on four-seamers (38.7 percent) with at least 500 fastballs thrown.

Hader's fastball is something of a ghost pitch: It's very difficult to hit but not for any of the usual reasons. So what makes it so effective?

Players who have faced him may have some insight. Dodgers infielder Max Muncy, who is 0-for-5 with four strikeouts in his career against Hader, told FiveThirtyEight that Hader's arm angle makes all the difference.

'When he's releasing the ball, it's almost underneath his armpit, and so when he has a high-spin fastball from that angle, it really does look like it's coming from the ground up,' Muncy said. 'And then he's throwing 97, 98 [mph] so it's just very, very hard to get on top of that fastball.'

Muncy is right about Hader's angle: Among lefties to have thrown at least 100 four-seam fastballs by July 14, Hader had the fourth-lowest release point but the 11th-greatest average velocity, according to Baseball Savant data.

But what about Muncy's contention that Hader has a high-spin fastball? After all, Hader's total fastball spin is below average. And less spin should mean less of the force in physics known as the Magnus Effect, which pushes up on a fastball, giving it the appearance of rising.Though it's really just dropping less than a fastball with less spin.

The explanation may be his spin efficiency.

There are two types of spin: transverse spin, which is sensitive to Magnus Effect and is what makes breaking balls curve and fastball rise, and gyroscopic spin or bullet spin, which is what makes footballs fly in a spiral and is not affected by Magnus force. The problem with using Statcast's raw spin total to evaluate pitches is that it combines both types of spin. What a pitcher cares about when trying to create movement on a pitch is his efficiency — or how much of his total spin is transverse spin.

To create perfect, 100 percent spin efficiency, transverse spin requires an axis perpendicular to the direction of velocity, while gryo spin moves parallel to that direction. (Again, think a football spiral.)

High spin efficiency likely explains Hader's amount of vertical movement despite his low total spin. It suggests that the majority of Hader's spin is transverse spin — which would explain the vertical movement. That's why he can blow his fastball right by the best hitters in the game even when they know it is coming, even when it's thrown right over the plate.

But creating such spin efficiency and vertical rise from such a low arm slot is unusual. Pitchers with lower arm slots typically release a ball with a more vertical axis that creates more side spin, like a sweeping slider from a lower arm slot. That's in contrast to over-the-top motions that typically create an axis more parallel to the ground given the nature of their hand position.

For example, Chris Sale throws an average four-seam fastball from nearly the same arm slot (5.45 feet above the ground) as Hader (5.19 feet). But Sale's pitch is thrown with a 126-degree axis, while Hader's is thrown with a 147-degree axis, according to Brooks Baseball estimates, putting Hader's axis closer to level.Spin axes pulled from TrackMan are estimates.

For context, Blake Snell has the fourth-highest release point for a four-seam fastball in the majors among lefties, owns a 160-degree spin axis on the pitch (more level) and ranks sixth among all lefties in vertical movement (10.5 inches). Despite Hader's much lower release point, their fastballs move in a similar way. In essence, Hader has Sale's release point but Snell's fastball. Somehow it seems that Hader is able to release a ball with an over-the-top grip and/or wrist position from a side-arm slot.

'Normally that arm slot is going to create a different spin angle,' Muncy says of Hader. 'That's what makes him unique. He's able to generate backspin from that arm slot that's what creates such a huge advantage for him.'

What might also be helping him — and is more difficult to measure and quantify — is deception. Hader's delivery can look like a whirl of skinny flailing arms and legs, and all that commotion can fluster opposing batters. If the opposing hitter needs a fraction of a second longer to pick up the ball out of the delivery, it's an advantage for the pitcher.

Hader told FiveThirtyEight that when he entered pro baseball, he weighed only 135 pounds.

'For me as a young guy, just being very small, I needed all the help I could to get my arm to throw faster and harder,' Hader said of his funky delivery.

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Some coaches in his amateur career and early in his pro career suggested that he change his awkward throwing motion, but the Orioles (who drafted him) and later the Brewers each had Hader undergo biomechanical evaluations, which found that his mechanics place below-average stress on his elbow and shoulder.

'This just came naturally for me,' Hader said. 'I think everyone is different. Everyone has a different type of arm slot and feel.'

As for explaining his success?

'I think it's a little bit of everything,' Hader says.

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It all adds up to an offering that has become one of the most unhittable fastballs of the pitch-tracking era.

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