Please excuse the cliché metaphor: analyzing pitchers is akin to peeling an onion. On the surface, we can use stats like strikeout-to-walk ratio and ground ball rate to analyze a pitcher’s results. But if we want to dig deeper into the how or why of the results, we need to peel back different layers. One level deeper is usually marked by examining the pitcher’s pitch repertoire and then, perhaps, the characteristics of an individual pitch. The bravest souls will continue peeling deeper, entering the murky analysis of pitch sequencing or pitch tunneling. Each layer that’s peeled back adds some amount of nuance to any conclusions reached.

In his five starts after joining the Mariners in September, Mike Leake posted an impressive 13.50 strikeout-to-walk ratio alongside a 2.25 FIP. That dominant performance wasn’t completely out of character for him—he posted an even better FIP across five starts in April—but it was pretty surprising. If anything, it was proof that there was still some untapped potential for the 30-year-old starter.

It’s been mentioned elsewhere but it bears repeating: Mike Leake’s slider is a pretty good pitch. The rest of his arsenal is rather average. He relies on command and guile to maximize his five pitch repertoire. But the whiff rate he’s able to generate with his slider is 25% better than the league average for that pitch type, easily topping the whiff rates on his other four pitches.

So what exactly makes Leake’s slider so good? In the table below, I’ve taken the component characteristics of the pitch and calculated z-scores for each of them. These z-scores will tell us how unique each characteristic is, based on the league average and standard deviation for major league sliders.

Slider Characteristics Pitch Characteristic Raw Value Z-Score Pitch Characteristic Raw Value Z-Score Velocity 81.4 mph 1.2 Spin Rate 2476 rpm 0.9 Horizontal Movement 3.5 in 0.7 Vertical Movement -1.1 in 1.5

A quick note about the data above: I’m comparing Leake’s slider to all sliders thrown more than 100 times in 2017. Because of the relationship between spin rate and velocity, the sample I’m using there only includes sliders thrown within two miles per hour of Leake’s average velocity.

Leake’s slider is thrown much slower than average, almost bordering on slurve territory. But despite the low velocity, the spin rate on the pitch is higher than its peers. The movement profile of the pitch is pretty unique too. It has a lot of horizontal break and a ton of vertical drop, again, almost looking like a curveball.

Beyond just the raw characteristics of the pitch, I think there’s something else that’s benefitting the effectiveness of this pitch. Last season, Leake shifted his release point by about a foot. It’s not as drastic as a change in arm angle, but it’s really apparent on the graph below. That little cluster of data points off to the left are all from 2017.

Leake’s slider (and cutter) actually lost some horizontal movement with the change in release point. But even though the shape of the pitch might have changed, it’s still continued to be an effective pitch for him. What gives?

What’s most interesting about this change in release point is its effect on Leake’s ability to tunnel his pitches. If you’re unfamiliar with pitch tunneling, it’s a fairly easy concept to grasp. The argument goes that a pitcher is far more effective if he can mask the movement of his pitches as long as possible, reducing the time batters have to recognize the pitch type. Within the past year, our ability to measure and quantify pitch tunnels has grown by leaps and bounds thanks to Baseball Prospectus. Because pitch tunneling inherently deals with pairs of pitches thrown together, it also gets us one step closer to being able to analyze the effectiveness of pitch sequencing.

For this article, the specific measurement I’m working with is Pre-Tunnel Maximum Distance (PreMax), defined as, “the distance between back-to-back pitches at the decision-making point.” This is the perceived difference in distance between two pitches at the point when a batter needs to make a decision to swing or not. The league average PreMax across all pitch pairs was 1.54 inches and a smaller PreMax value is better. Because of the difference in perspective, pitch pairs are further divided by batter-handedness as well.

Leake will most often play his slider off his sinker and his cutter, though he will use back-to-back sliders against right-handed batters on occasion as well. Below is a table of his PreMax values from 2017 for his five most common pitch pairs ending with a slider.

Pitch Tunnel Pairs Batter Handednes Pitch Pair PreMax Count Batter Handednes Pitch Pair PreMax Count RHB Sinker-Slider 1.36 111 RHB Cutter-Slider 1.41 58 RHB Slider-Slider 1.31 45 LHB Sinker-Slider 1.31 30 LHB Cutter-Slider 1.22 31

Each of those PreMax values are well below the league average (a good thing). Most importantly, his sinker-slider pair was the best it’s ever been last year. It was a full standard deviation better than league average. His sinker breaks in on right-handed batters while his slider breaks away from them, but he was able to mask these two pitches excellently last year. Perhaps it was the difference in horizontal movement on his slider that helped him tunnel his pitches better.

Using Baseball Prospectus’s new interactive matchup tool, I found a few prime examples of this pitch tunneling effect. (Unfortunately, MLB.tv’s archive was not working for me so I don’t have any gifs of these examples. :( )

9/6/17 vs. LAA

Batter: C.J. Cron

This pitch pair came after Leake fell behind 2-0. The third pitch was a sinker taken over the heart of the plate. He came back with a slider below the zone that Cron swung through. Leake finished Cron off with a slider in almost the exact same location, getting another swing and miss for the strikeout. The first slider was masked extremely well after Cron had seen three sinkers to start the at-bat.

9/24/17 vs. CLE

Batter: Jay Bruce

Leake started off this at-bat with a curve to steal a strike. Bruce fouled off the second pitch, a cutter. The third pitch continued the trend, as Leake located a slider inside and below the zone, getting the final swinging strike for the out. This pitch pair was even better than the one to Cron above. Leake’s cutter-slider sequence to left-handed batters is particularly deadly.

We’ll have very little pitch tracking data until the regular season starts—Peoria turned off their PITCHf/x system this year. Once we get that data, we’ll know pretty quickly whether or not Leake has maintained the adjusted release point he debuted in 2017. Given the effectiveness he’s shown with his slider in the past, his improved ability to tunnel this pitch with his sinker and cutter could help him maintain the higher strikeout rate we witnessed in April and September. That’s the kind of upside the Mariners were hoping for when they traded for Leake late last season.