Swing Softly And… Wait, No, That’s It. Just Swing Softly.

Ever since Major League Baseball released the first drip of its bat tracking data this spring, I’ve been keeping an eye on a particular leaderboard. I don’t know if anybody else cares much about it, but I’ve been fascinated by the fast swing leaderboard. I haven’t been tracking it religiously; I’ve just been checking in every couple of weeks. Also, I haven’t been looking at it the way you’re supposed to. I’m only interested the bottom of the list. I suppose that makes it not so much a leaderboard as a trailerboard, but I don’t care. I’m interested in it because there’s an honest-to-goodness horse race going on there.

A fast swing is one where the barrel of the bat is traveling at least 75 mph when it strikes or comes closest to the ball. That number was chosen, per Mike Petriello, “because that’s the line where, on a per-swing basis, a swing goes from negative run value for a hitter to average, on its way to positive.” All things being equal, it’s better to swing hard. On an individual player basis, here’s the correlation between fast-swing rate and wRC+. Roughly speaking, five percentage points of fast-swing rate is worth three extra points of wRC+:

Here’s something that may surprise you: Fast-swing rate (R = .48) has a stronger correlation to wRC+ than average bat speed does (R = .41). I assume that this is the case for the same reason that 90th-percentile exit velocity is a more useful stat than average exit velocity. You’re ignoring a big chunk of less useful information and focusing on the swings that can result in real damage.

To be clear, there’s a certain kind of bias baked into these numbers. It’s not just that fast swings are better, it’s that most players need to put themselves in good position to get off their best swing. The more you have to adjust to the pitch because you didn’t time it up right or because it moved in a direction you weren’t expecting, the more you have to change your mechanics, which will drag down your bat speed. Therefore, players with better pitch recognition, better plate discipline, and more adaptable swings will get off harder swings more often. The highest bat speed recorded so far is 88 mph, but 25 different players have hit at least 87.7 mph, and 67 different players have hit at least 87.0. In other words, lots of players are capable of hitting the ball as hard as Aaron Judge or Shohei Ohtani. The real differentiator is their ability to get to that power on a regular basis.

The bottom of the list works in a similar fashion. There are plenty of players who rarely swing hard, but they have different reasons for it. Few of the players at the very bottom of the fast swing leaderboard have the capacity to swing as hard as Oneil Cruz. Still, some are intentionally taking shorter, softer cuts because they’re prioritizing solid contact, while some (I’m so sorry, Charlie Blackmon) seem to be swinging softly because they just can’t put themselves in a good position to drive the ball. Today, we’re interested in the two players who best exemplify the soft-swing-to-celebration pipeline: Luis Arraez and Steven Kwan.

I’ve been checking the bottom of the fast swing leaderboard for one reason: For most of the season, neither Arraez nor Kwan had registered a single fast swing. They were at 0.0%. They were the only qualified players with double zeroes, and I was rooting hard for them to make it through the whole season that way. Through July 9, Kwan was running a 172 wRC+, fifth best in baseball, while Arraez was at 104. The leaderboard was beautiful and it looked like this:

Alas, nothing gold can stay. On July 10, Arraez saw a juicy 96.5-mph four-seamer from Bryce Miller right over the middle of the plate. The Crown Prince of Cromulent Contact finally caved to his inner caveman, unleashing a 77.1-mph swing that resulted in a 97.9-mph bullet — right to second base for an easy groundout:

And just like that, it was a one-horse race. Despite coming from nowhere to slug .512 in the first half, Kwan stood alone as the only qualified player in baseball who hadn’t registered a single fast swing all season. Maybe that pressure was too much for him, because he only lasted another 13 days before joining Arraez. (More likely, Kwan was simply pressing, as he was at that point mired in one of the worst slumps of his short career.) Like Arraez, Kwan yanked a harmless grounder to the right side of the infield. Unlike Arraez, he didn’t even make great contact, sending a Shelby Miller splitter at the bottom of the zone to first base with a 76.5-mph swing:

A little over two weeks later, Kwan lapsed again, but at least this time he got his money’s worth. Simeon Woods Richardson put an extremely hittable four-seamer belt high and right over the middle, and Kwan acknowledged the offering with the bare-minimum fast swing, a 75-mph cut that sent the ball 396 feet into the right field bleachers:

Although that was the fastest tracked swing of Kwan’s season, it only ranked 20th in terms of exit velocity. That’s one of the tricks that both of these players possess. They’re so good at squaring the ball up that they don’t necessarily need to swing hard. Back when bat tracking data came out, the formula for determining exactly how squared-up any batted ball is was released as well. It’s a simple percentage based on the speed of the pitch and the speed of the bat. It looks like this:

Squared-Up Percentage = EV / ((Bat Speed x 1.23) + (0.2116 x Pitch Speed))

Using that formula, let me hit you with a trick question: Every time Player A swings, they have a bat speed of 65 mph, and they square the ball up at 90%. Every time Player B swings, they have a bat speed of 75 mph, and they square the ball up at 80%. Who hits the ball harder?

I told you this was a trick question. They’re nearly identical. Player A hits the ball 91.0 mph, while Player B hits it 90.7 mph. Keep in mind that the squared-up cutoff is 80%. Keep in mind that an average bat speed of 75 would put you near the top of the league and 65 mph would put you near the very bottom. Every single time, Player B is squaring the ball up and swinging with the bat speed of the average home run, but they’re still a hair below Player A. Kwan and Arraez are never going to challenge for the league lead in exit velocity, but they square the ball up both often and well.

I’d like to end on something that’s definitely not statistically significant, so don’t take it too seriously, but I still think it’s pretty interesting. At the beginning of this article, I mentioned that 75 mph is the cutoff for a fast swing because, on average, that’s where swings start to accrue positive run value. It takes a humongous sample, like entire-league humongous, to make those numbers statistically significant. On an individual player basis, any correlation you find will be tiny, but I decided to look at every tracked swing this season from both Arraez and Kwan, and then calculate the correlation coefficients between their bat speed and run value. For Arraez, there was almost no correlation whatsoever (R = .01) between bat speed and run value. Even if you limit the sample to balls in play, the correlation only increases to .02. For Kwan, the correlation is still an extremely weak .06, but it at least seems to exist. Out of curiosity, I ran the numbers for Aaron Judge, and found a correlation of .09, going up to .13 if you look only at balls in play. For Judge, harder swings tend to see better results. For Kwan, and to a shocking extent for Arraez, the two are much less related. Since they don’t necessarily need to swing hard to perform well at the plate, maybe it’s time they focus on what’s most important. Right now Arraez is ahead 1-to-2 in the Hard Swing Avoidance Sweepstakes. He’s got just over a month to defend that lead.