When the Houston Astros sent two of their top pitching prospects, Vince Velasquez and Mark Appel, to the Philadelphia Phillies for Ken Giles in December 2015, they were expecting an excellent flame-throwing reliever, and possibly their closer of the future. In his first two years in the league, Giles amassed a 1.56 ERA in 115.2 innings. His work as a setup man/closer went largely unappreciated due to the losing nature of the Phillies, but Giles pitched like one of the best relievers in the game.

But his first year in Houston did not go as planned. Giles couldn’t maintain a hold of the closer job, as he blew five of his 20 save opportunities and finished with a 4.11 ERA. His 2.86 FIP proved he might have suffered from bad luck, and he still displayed incredible stuff (nearly 14 K/9), but he did not execute as expected or needed for the Astros.

2017 has been a different story for Giles. His 29 saves rank ninth in the league, and he has blown only three opportunities this season. His 2.30 ERA is legitimate, supported by a 2.14 FIP. Giles has been one of the best closers in baseball, but his name is hardly mentioned among the top guys in the league. And he’s been especially locked in of late.

Last night (September 5th at the time of writing this), Giles struck out the side in a 10-pitch inning to earn the save against the Seattle Mariners. The only ball he threw came when the batter barely checked his swing. Here he is hitting triple digits on the outside corner to get Ben Gamel looking and close out the game:

It was the second night in a row that he struck out the side for 1-2-3 ninth.

Since June 7th, Giles has a minuscule 0.86 ERA and .147 average against. FIP will rarely support a mark that low, but his 1.57 mark in that category is still exceptional. He’s striking out more batters and walking fewer, accumulating a K-BB% of 31.3%.

Giles has given up one run since July 16th, in 20.1 innings of work. His FIP is under 1, at an absurd 0.82, and he’s sporting a ridiculous 45.1% K-BB% in that time. He has also show the ability to be stretched out of late, as he has gone 1.2 or more innings in three of his last ten appearances. What has made him so effective this season?

It all starts with the slider for Giles, which ranks third in run value among relievers at 12.9 runs. Run values aren’t the best metric, but they definitely give you an idea of the effectiveness of a pitch. Just look at it:

The pitch starts at the “TEXAS” on Rougned Odor’s jersey and finishes below his knees. There is about nothing a hitter can do with that.

Look at a heat map, by pitcher viewpoint, of Giles slider’s location. He is burying the majority of his sliders along the bottom of the zone. Now look at a heat map of the average against the pitch, by zone. Where the majority of the pitches are going, hitters aren’t doing much with. At all. Per Brooks Baseball, hitters have only put the ball in fair territory on 25% of their swings at the pitch. They rarely put the ball in play, and they don’t do much with it when they do.

But this is actually not new for Giles. He was third last year in slider run value among relievers, at 12.6 runs. Where Giles has greatly improved his effectiveness is with his fastball. It was worth a run value of -13.3 in 2016, but it’s currently sitting at 3.4 this year. It has not been incredible, but paired with the slider, it doesn’t need to be.

Here is a comparison of his fastball in 2016 vs. 2017:

Season AVG OPS xwOBA Zone% Contact% SwStr% wRC+ 2016 0.376 1.079 .415 53.6% 85.5% 7.1% 200 2017 0.286 .829 .330 59.1% 77.3% 11.8% 137

The batted-ball numbers are down across the board. He’s throwing it in the strike zone more often as well, which would cause you to expect he is pitching more to contact with the pitch. However, the Contact% has steeply declined, and the swinging-strike rate is way up. Obviously, with a 137 wRC+ allowed this year, the pitch is still not great. But when you have a slider running a -14 wRC+, it does not need to be.

Here is a heat map comparison of the two pitches: 2016 vs. 2017

The spray is much tighter in 2017, and he is throwing across the middle of the zone a whole lot less. Improved command of a pitch will obviously lead to more success. But another element might be involved in the improvement of his fastball. Giles has added nearly four inches of horizontal movement this year, from -1.7 to -5.6.

A 2016 fastball:

And the fastball from last night again:

The run to the right on the pitch is clear. Movement of any kind will always help to keep a hitter off balance, and while we can’t be sure, it looks like this may be what has given life to Giles’ fastball. His confidence with the pitch has grown, as his usage of it exploded from roughly 50% to 68.4% in August. And it appears this improved fastball may be keying his emergence as one the best closers in the game.

Giles lost some respect and notoriety with his poor 2016. But with the year he has put together so far, especially the way he’s pitching of late, he has earned all of that, and then some, back. He’s locking down the back of Houston’s bullpen. The Cleveland Indians displayed the importance of relievers in the playoffs last season, so don’t be too surprised if Giles is at the forefront of a charge to the World Series for the best team in the American League.