Taking a Look at Spin Mirroring

There are a myriad of things pitchers can do to get a leg up on a hitter. Changing speeds, eye level, sequencing, and even spin rate are a few of the more popular methods. Changing speeds can interfere with a hitter’s timing, while changing eye levels can force a hitter to adjust to a larger focal point. Varying pitch selection based on the situation can help a pitcher become less predictable, and changing the spin rate can have an effect on the expected movement of a pitch.

But there are other subtle tricks pitchers have up their sleeves. One of these tricks, explored in an article for The Athletic by Joe Schwarz and elaborated on in another by Eno Sarris, is known as pitch or spin mirroring, and with the right pitch attributes, it can be a powerful weapon.

Being able to spot a pitch’s spin can tip a hitter off to what is coming, a skill some hitters claim to have. That’s a big advantage to have in a decision that transpires over the course of milliseconds. When pitches are spinning over 2000 times a minute, is the human eye really that good? Perhaps. As Preston Wilson points out in the piece linked to above, a hitter might see “more white or more red,” which gives an indication as to what pitch is coming. More white would indicate a faster or more abundantly spinning pitch, like a curveball or fastball, while more red could be a changeup.

In theory, a pitcher could use spin mirroring effect to parry those abilities, especially with a fastball and curveball combo because of the high spin on both pitches. Furthermore, if the rotation blur of the ball is mostly white, you’ll have a much harder time deciphering the direction in which the spin is oriented.

Attempting to gain a leg up on eagle-eyed hitters is only one way to take advantage of opposing spin directions. Being able to tunnel two pitches with mirror axes sets up the potential to create large separation ratios from the commit point to the plate. A hitter thinking they’re about to see a fastball but getting a curve instead might result in a really foolish-looking whiff or, if the hitter is able to adjust enough to get close to the breaking ball, a weakly-hit worm-killer.

To understand this pitching philosophy, you’ll need a basic understanding of pitch physics, mainly the concept of Magnus force. You can read up on this phenomenon here, but I’ll break it down as best I can.

With a fastball, which exhibits backspin, Mangus force will work in such a way that keeps the ball elevated against gravity, though of course, gravity will eventually win. The a high spin rate (as well as velocity) will give the illusion that the pitch is rising, although it’s just staying the course longer than our brains are expecting it to. With topspin pitches like curveballs, Magnus force pulls the ball down, as the spin direction is creating force with gravity.

The idea is to pair a fastball with a non-backspin oriented pitch and allow their spin directions to work off each other in a way that’s beneficial to the pitcher. Curveballs work the best by virtue of the fact that most traditional versions are topspin-heavy. Since the 180 degree spin contrast will push the pitches in opposite directions, a fastball and curve have the potential for a large location spread at the plate to create a difficult swing dilemma for the hitter.

Here’s an extreme example using Stephen Strasburg throwing his four-seamer and curveball. Thrown back-to-back, the pitches track together for a short time, then split a little before the commit point. Since Strasburg was able to mirror his axes during this sequence, Corey Seager took a bad swing at the curve:

Seager may have thought he was getting another fastball since the pitches were tunneled for a moment and then took drastically different routes to home plate.

There is greater gyro spin involved with other pitches, such as a slider, making it much harder to create that mirror effect with a fastball. You’re more likely going to have something close to a 90 or even 45 degree axis differential, which can create varied movement splits and be effective in their own way. For sliders, the best option for spin mirroring might be a changeup. The spin direction of a changeup, which (for right-handed pitchers) is typically oriented to 9:00 while a slider is more likely to be oriented at 3:00.

The 90 or even 45 degree axis contrasts are useful but mostly in a platoon split situation. If you’re a southpaw and you throw a fastball and slider that repel at 90 degrees with the slider breaking away from a right-handed hitter, this option would be best suited for that split as it might not be as effective against lefties. Thus, the fastball and curveball would be the superior combo because it can work against hitters on either side of the plate.

Here is a good example of how to design a slider and changeup to work off of each other with as close to a 180 degree spin axis contrast as possible. Ball or strike be damned, this is tremendous movement from Sergio Romo’s slider (2:45) and change (8:30).

But since most pitchers throw a fastball, we’ll take a look at how well sliders mirror versus how well a curveball does.

I gathered the top five slider and curveball values of 2019 for pitchers who threw at least 80 innings and also threw each pitch at least 20% of the time. There are varied classifications on some of these breaking pitches, which make them more than simply a slider or a curveball, but for argument’s sake, we’ll pigeonhole them under their applicable umbrella. I’ve also included Frankie Montas, who throws a two-seamer/sinker more than a four-seamer; I will be using his pitch data for that pitch here.

Here are the major’s top five valued sliders for the 2019 season. Included in the list is the average spin direction for both pitches, with the tilt differential added to the end:

Before I show some examples, I should note that I tried to get pitch combinations that reflected the average spin direction and location, as well as pitches that were called strikes or were whiffed at.

Let’s look at Liam Hendriks first, who has the smallest spin axis difference:

It’s actually not that bad. We see tight pitch paths and a pretty good tunnel, but a hitter with a keener eye than Chris Davis‘ might be able to tell the difference between these two pitches. The results could have been much different had Davis let the slider go and cut at the four-seam.

Here’s an example of Scherzer’s four-seamer and slider, our largest differential and the closest to the 180 degree mirror:

Much like with Hendriks, we have a really tight tunnel but a much larger eventual spread. Once the pitches break the commit point, you can see the axes working off of each other.

Now let’s look at curveballs with the same criteria as the sliders:

As mentioned, the curveball pitchers above do a much better job mirroring the fastball. Is it a coincidence that four of the top five rated curveballs all are within eight degrees of the 180 degree goal?

Let’s start with Morton, whose curveball and fastball averaged an 183 degree axis differential. In this up-close look, you can see how the spin direction on both pitches mirror each other:

And here’s Nola, who averaged four degrees off the suitable differential, working his knuckle curve and fastball in tandem. Bear in mind that this angle skews the true mirroring of these pitches, but you can see the kind of separation they can induce:

Given the natural oppugnant spin direction of fastballs and curveballs, there are many pitchers who have the potential to mirror their curveball and fastball. There is plenty of evidence (some of it admittedly anecdotal) that demonstrates its potential usefulness. We saw in the curveball chart that the most effective curveballs came really close to a perfect mirror with of the pitcher’s fastball. Coaches and trainers should take more time to observe how their pitchers’ fastballs and curveballs play off each other, and explore options and adjustments to properly mirror this combination.