We knew we had to pay attention to Jordan Hicks from the moment he set foot in the Major Leagues. His very first pitch, a sinker to Brandon Nimmo on Opening Day, arrived at 100.3 mph.
Hicks threw seven fastballs that day, all between 99.7 mph and 101.6 mph. He hasn't stopped throwing heat since, topping out at a nearly unbelievable 105 mph on May 21. Hicks has nearly half of all 100 mph-plus pitches thrown across the entire big leagues in 2018. His 99.7 mph average fastball velocity is easily baseball's best.
That's impressive for anyone. It's especially so for a 21-year-old who was in Class A last year, who was the team's first Grapefruit League cut this spring, and who somewhat stunningly made the Opening Day roster nonetheless, in part due to injuries to Luke Gregerson and Adam Wainwright.
Thanks to all that lightning in his arm, Hicks was one of baseball's biggest early stories. All that velocity served to mask a pretty important truth, however: He wasn't actually a productive pitcher, at least not to the level the velocity would have indicated. Hicks showed a near-stunning inability to miss bats. As recently as June 3, he still had as many walks as strikeouts. Velocity is an important tool, but it doesn't by itself make you good.
That's beginning to change. Over the past three-plus weeks, Hicks has legitimately been one of the better relief pitchers in baseball. He's no longer a one-tool curiosity. He's a big part of a contender's bullpen.
So what changed? It's not about the fastball. It's about what Hicks' slider does along with it.

Let's back up for a second and break Hicks' season into two parts, roughly the first two-thirds (through May 21, including his first 21 games) and the most recent one-third (since May 22, including 10 additional games).
The first two-thirds (21 games through May 21)
Through most of the first two months of Hicks' rookie season, these were his important numbers.
22 innings
9/16 strikeout/walk
9.5 percent strikeout rate
2.05 ERA
4.88 FIP
.354 xwOBA
Let's quantify what some of those numbers mean. Through May 21, there were 244 pitchers who had thrown 20 innings. That 9.5 percent strikeout rate was the second-lowest. It was tied with Chris Tillman and Bryan Mitchell, who had ERAs of 10.46 and 6.64, respectively. It was ahead of only Texas' Alex Claudio, who throws one of baseball's softest fastballs (86 mph). 
Expected wOBA, or xwOBA, is an advanced Statcast™ metric that displays the quality of and amount of contact (and walks), expressed on a scale similar to OBP. Since the league average is .331 and Hicks' mark was .354, it's shown here to point out that the otherwise-shiny 2.05 ERA was wildly misleading. Based on the strikeouts, walks, and quality of contact, Hicks was pitching like a pitcher with a 5.50 ERA.
The second one-third (10 games since May 22)
13 innings
19/3 strikeout / walk
40.4 percent strikeout rate
2.08 ERA
0.87 FIP
.218 xwOBA
Now we're talking. Entering play on Wednesday, 189 pitchers had thrown 10 innings since May 22, and Hicks' 40.4 percent strikeout rate is the fourth-best. He's gone from being almost unable to strike hitters out to looking like one of baseball's best at it in the blink of an eye. That .218 xwOBA not only supports the 2.08 ERA, it's a Top-15 mark.
So how did he make that happen? It's not more fastball velocity. It's not different pitches; he's the same 75 percent sinker / 25 percent slider he's been all year. Instead, it's about where the slider is -- and how hitters react to it.

If Hicks had a problem early in the year, it was this: He had only two pitches, and one of them wasn't competitive. In April, 175 pitchers threw at least 50 sliders, and only four got fewer swings than Hicks' 28.6 percent. It was worse outside the zone, which is where you really want sliders to induce swings. Only two pitchers got fewer chases on sliders than Hicks did.
That left Hicks with a pretty big problem. Batters simply weren't fooled by his slider, refusing to swing at it. That allowed them to sit on Hicks' fastball, and that was all he had. While the velocity and movement allowed him to avoid a ton of loud contact -- he's got a Top 10 ground-ball rate, at 59.9 percent -- this is where the lack of strikeouts and the excessive walks came from. Hicks was a one-pitch pitcher, and that one pitch was built for grounders, not whiffs.
While Hicks is throwing his slider harder -- it was at 83 mph in April, and it's 87 now in June -- this is really a story about making it enticing to hitters.
Hicks is throwing his slider to the edges of the zone (defined here as being a baseball's width on either side of the defined strike zone) far more these days. It was 21 percent in April, 28 percent in May and 42 percent in June. That's an enormous increase.
All of a sudden, hitters can't spit on it and wait for the fastball. In June, Hicks is getting nearly 60 percent swings on the slider, when it was below 30 percent in April. And because it's a good pitch, when they do swing at it, they rarely make contact against it. He may have only thrown the slider 19 times so far this month, but he's already got six strikeouts on it.

Now, not only does Hicks have a viable secondary pitch, it's making his fastball look better, too. His fastball had a five percent strikeout rate in April, a seven percent strikeout rate in May… and a 40 percent rate in June.
What this all means is simple. It means that Hicks is no longer just "the guy with the fastball" and nothing else, the one-pitch pitcher who wasn't necessarily all that productive. Now that the slider is forcing hitters to have something to think about, it's making the fastball play up, too. Hicks might only have two pitches, but when you can throw 105 with a slider like that, it might be all you need.
Over the past few weeks, Hicks' production has finally caught up to the hype. For a troubled St. Louis bullpen, it's the best news they could have received.