Full, immediate disclosure: it’s actually a tie. When you think unhittability, you might think batting average against, or slugging percentage against, or something. Me, I prefer contact rate, because it’s elegant and simple. Contact rate measures how often batters hit the baseball when they attempt a swing. Most simply, that’s the whole point of a swing. A pitcher who gets a lot of missed swings can rightly be said to be unhittable. Other metrics might penalize for wildness, or just take other matters into account. For pure unhittability, I like looking at the rate of contact, and among National League starters, the lowest rate of contact allowed partly belongs to Francisco Liriano, at a hair above 71%.

Liriano’s tied with somebody else, and it’s not Matt Harvey or Clayton Kershaw. I’m going to be writing about that somebody else, because Liriano’s a lot more familiar. Liriano’s long been hard to hit, and he’s had a breakthrough season for the Pirates after coming over on the cheap. Don’t get me wrong, Liriano’s been surprising, but the guy with whom he’s tied has been a bigger surprise after also joining his current team on the cheap. I’m not sure even the Padres knew what they were getting when they dealt for Tyson Ross.

So far this year, 179 pitchers have started at least ten games. Of those, no one’s allowed more frequent contact than Vance Worley. No one in the NL’s allowed more frequent contact than Clayton Richard. At the other end of the leaderboard, here are the top ten lowest contact rates given up:

Ross, for whom the Padres traded Andy Parrino and Andrew Werner. Ross has started 14 games, totaling 80 innings in those starts. Before the year people weren’t even sure if he’d be a long-term starter or reliever. Since the All-Star break, no starter in baseball has allowed a lower rate of contact. This year overall, Ross is tied for the NL lead, missing bats more often than the league elite.

Because of the low rate of contact, Ross generates a good number of strikeouts. He’ll never be confused for a pitcher with great command, but he offsets that by keeping balls in play on the ground, and the end result is that Ross — as a starter — has posted some outstanding numbers. His immediate company this year, by FIP-:

Francisco Liriano

Chris Sale

Cliff Lee

His immediate company this year, by xFIP-:

Ross was and is known as a tall righty with a funky delivery that seems to generate next to nothing in the way of momentum. He’s still the same guy, but now he’s flourishing completely off the radar, because the Padres exist off the radar, particularly after they drop out of the race. Ross has quietly gone about pitching like a staff ace, in a rotation that, for a while, looked like one of the worse rotations all-time.

So, the story? Let’s look at a couple images, showing Ross’ pitches against right-handed batters and left-handed batters:

Against righties, you see a lot of whiffs down and/or away. Against lefties, you see a lot of whiffs down and/or in. Pair that with the following information:

vs. RHB: 68% contact, 64% strikes

68% contact, 64% strikes vs. LHB: 75% contact, 58% strikes

This paints the picture of a slider specialist. For further evidence, let’s look at all seven of Ross’ strikeouts against the Pirates in his start on Wednesday:

Those are in no particular order, because the order doesn’t matter. What matters is that all seven strikeouts came on two-strike sliders. The righties went after sliders. The lefties went after sliders. The slider is Ross’ special pitch, much as is the case with Patrick Corbin, and while you expect that to be too little for a starter, it hasn’t been too little for Ross, not yet. Among starters, his slider rate is the fifth-highest, and second-highest out of righties, behind only Ervin Santana. Ross has not thrown much of his changeup. He’s been outstanding, even getting up to and around 100 pitches.

With two strikes on lefties, Ross has thrown his slider about half the time. With two strikes on righties, Ross has thrown his slider more than half the time. In this way, the slider is responsible for the overwhelming bulk of his strikeouts, and only Corbin’s slider has missed more bats. In fact, the Baseball Prospectus PITCHf/x leaderboards allow us to look at all the various pitch types and their results. Out of all the individual pitches starters have thrown at least 200 times, Corbin’s slider has been the single most unhittable pitch. Ross’ slider has been the second-most unhittable pitch. Batters have whiffed a little more than half the time they’ve offered.

As for the quality of contact, when contact has been achieved? That slider has yielded a dozen singles and a double. By isolated power, we’ve got a .007 mark. There’s a .186 BABIP. Ross has been unhittable in large part because of his slider, and even when his slider has been hit, it’s been hit poorly. That’s an effective slider, Ross has been throwing.

So, two questions:

What the hell? Now what?

How did Ross turn into this, after he was all but discarded by the A’s? Obviously, there’s the matter of the league switch, but it also stands to reason Ross might have made some mechanical adjustments. On that:

Ross reaches back to spring training to pinpoint the origin of his breakthrough. In Peoria, Ariz., Balsley prescribed a “step-through drill,” the idea being to sync Ross’ foot strike with the moment his arm went up. “He had me walking into throws off the mound and just got the timing synced up right,” said Ross, who despite standing 6-foot-6 has the shortest stride length of any Padres pitcher. “That was always my problem, having the leverage just a little bit off with my timing. But we figured out a way to get it all in sync, and everything’s just flowed from there.”

Maybe that’s it, maybe that’s not. Ross says that’s it, but whenever a player is questioned about a breakthrough, he’ll search for some kind of answer, some kind of turning point. It’s easy to see how maybe Ross’ delivery now might be more consistent. It’s easy to see how that would pay off. This is one possible explanation, out of a handful of potential explanations. There’s no question that Ross has always had powerful stuff.

As for the career path from here? Even after 14 starts, that’s just 14 starts. We’re still programmed to believe that starters need three good pitches, or at least a less platoony second pitch than a slider. Ross does possess a changeup, and though he’s not in love with it, it could be developing:

Padres right-hander Tyson Ross, who earned the win on Saturday against the Reds, has been working on a project throughout the season. Looking to expand his arsenal, Ross has added a changeup to go with his fastball and slider. Saturday marked Ross’ fourth start since moving back to the rotation out of the bullpen. Although he used the changeup sparingly in the win — his third in in those four starts — he said he’s becoming more comfortable with the pitch.

There’s no guarantee it’ll become a good pitch — everyone works on a changeup, and sometimes it just doesn’t stick — but the effort’s there to turn it into a good pitch, and Ross is still clearly trying to figure himself out. He’s far from a finished product, no matter what the numbers might suggest.

And those numbers do at least indicate that maybe a third pitch isn’t quite so necessary. Maybe Ross’ slider is just that good. He has himself a platoon split, as expected, but it’s not that lefties have killed him — it’s that lefties have had some success, while righties have been murdered. If your baseline level is good, then you can live with a slider-heavy platoon split. The slider, clearly, gets lefties to swing and miss. It’s also worth considering that there might be something deceptive about Ross’ delivery. The league average is that 42% of fly balls are pulled, and pulled fly balls tend to do the most damage. Ross is at 23%, allowed. Last year, he was also well below the league average. If batters aren’t picking the ball up until late, that puts them at more of a disadvantage, and that might help Ross offset the missing quality third pitch. This is me speculating, but the numbers shouldn’t be outright ignored.

Tyson Ross could stand to throw more strikes. He could stand to throw a better changeup. He could stand to have a more fluid delivery. But his current delivery works for him, he keeps balls in play on the ground, and though he’s mostly pitching like a reliever acting as a starter, he’s been one of the most unhittable starting pitchers in baseball. He’s been the most unhittable starter since the break, and he hasn’t lost any zip working out of the rotation. There was always stuff to like about Tyson Ross. The most we can say is that now there’s more.