Midway through the 2015 season, the back end of the Twins bullpen was a bit of a mess. Beyond Glen Perkins, the Twins were primarily relying on Blaine Boyer in the late innings, and though Boyer apparently had a solid season according to his 2.49 ERA, his secondary numbers showed a pitcher that was anything but dominant. Striking out only one batter every other inning and a low BABIP (.270, league average was .296 last season) showed a pitcher that was actually getting by with weak contact instead of actual dominance of hitters.
While Boyer solidified his role as the primary set-up man before the additions of Trevor May and Kevin Jepsen, another pitcher was seeing his stranglehold slip. Casey Fien had split duties with Jared Burton over the past couple years, but this season wasn't looking quite so good. Even though he was posting the best walk rate of his career, his strikeout rate had trended in the opposite direction. While he wasn't anyone's idea of a shutdown reliever, he did average a strikeout an inning over the past two years while walking virtually no one. However, this year his strikeout rate dipped below 6 K/9, which wasn't quite as bad as Boyer's but was pretty close.
The funny thing is that Fien survived in the same manner as Boyer, allowing a .272 BABIP that combined with his 1.14 BB/9 rate to keep runners off the bases. Fien's strategy for attacking hitters was pretty simple, as he was going to throw fastball after fastball at them.
Well, that's not entirely true. Sure, there was his straight fastball that averaged 92 MPH and would occasionally get pumped up to 95, but that was thrown just over half the time. An additional 37% of his repertoire was the weapon that Mariano Rivera made famous (or did it make Mo famous?): the cutter.
Even though Fien is a flyball pitcher and can be homer-prone at times, he's found success with his excellent control and a repeated ability to induce weak contact. Typically a pitcher's BABIP should stabilize around .300, and although Fien's 2015 BABIP was .272 as mentioned above, he's shown a consistent ability to run a sub-.300 BABIP ever since joining the Twins in 2012. That sustained success has likely come from messing with hitters' timing as he would alternate between his low-90s fastball and high-80s cutter.
As I was taking a look at Fien's cutter relative to his fastball, I did find something interesting. Typically we think of a fastball being straight or having a little tailing action on it, while a cutter will have movement that bores into a lefthanded batter when thrown from a righty. The pitchF/X graph from Wade Davis below exemplifies this movement.
The average 4-seam fastball from a righthanded pitcher had 8.75 inches of "rise" and 5.95 inches of tailing action, while the average cutter "rose" 7.18 inches and broke just under half an inch into a righthanded batter. Meanwhile, well, Casey Fien was a bit different.
The vertical difference isn't of as much interest as the horizontal difference. Fien's fastball is very straight, having under 2 inches of movement, so although his cutter (mislabeled as a slider here) has some pretty typical movement, his cutter drops nearly straight down relative to his fastball.
Below I've included two clips of Fien. The first one is his fastball and the second one is the cutter.
As you can see, his fastball seemingly has cutting movement, but that's partially a function of the camera angle, the pitch placement, and also that Fien's fastball simply doesn't tail as much as a typical fastball. Likewise, the cutter does seem to have similar horizontal movement in the clip (thank you Baltimore and New York for having similar camera angles) and you can also see the ball drop almost straight down ever so slightly. With such a small difference in movement between the two pitches, a hitter could be sitting on the fastball only to be dealt the cutter, causing the batter to make weak contact with the pitch. While the weak contact is good, it definitely would be better if Fien could find a way to rack up the strikeouts again to make himself a stronger option for the Twins in the 'pen.
Armed with these two weapons, Fien has been a solid reliever in the back of the bullpen for the past four seasons. With the addition of Kevin Jepsen, Fien will be more of a 7th inning pitcher and middle reliever next season, but that role suits him better, anyway. He is arbitration-eligible and MLB Trade Rumors is projecting him to earn $2.2 million next season, which with another solid season will end up being money well spent.