Outside of on-field performance, there’s a few things that will always endear me to a pitcher. Stirrups, check. Weird facial hair, check. Funky pitching motions, double-check. As a fan of the Giants and their farm system, sometime around 2007 Sergio Romo caught my eye when he completely devastated the California League. If you ever read any scouting report on Romo, they all sing the same chorus: deceptive motion, throws strikes, doesn’t blow hitters away with velocity. Deceptive motion? Now I’m intrigued. Almost all scouting reports made a note on Romo and his varying arm-slots and looks he gave hitters. He was mostly a fringe prospect, but for me, his ’07 in the California League is just too nuts to not impress.

Take a gander:

Yes, that’s what a 14.4 K/9 looks like. Romo struck out 106 in 66.1 innings while only walking 15 hitters. Funky motion, or not, you won’t see a better season from a pitcher’s standpoint anywhere. Romo was simply un-hittable for that year. Romo eventually made his MLB debut in 2008 at 25-years-old and, has since, been solid reliever for the Giants. In 79.2 career innings pitched at the major league level, Romo owns a 2.61 FIP that he’s paired with a 9.83 K/9 and a 2.15 BB/9. What’s been so vital to Romo’s success as a pitcher?

It’s an easy question for anyone to answer that’s seen Romo pitch: it’s his slider. By FanGraph’s pitch-type values, Romo’s slider has been worth 12.6 runs above average since 2008 — if you go by the 10 runs to 1 win rule, that’s a 1.2 win pitch for Romo. Romo possesses a frisbee slider that he throws from a few different arm angles and batters haven’t been able to touch the pitch since Romo entered the league.

The above graph indicates Romo’s slider usage this year split by batter handedness. Whiffs are represented by black circles and contact is represented by X marks. In 2010, Romo has thrown a total of 165 pitches — 67 of which have been the slider. Romo has thrown the slider 40% of the time this season, he’ll pair the pitch with a 2-seam fastball and the occasional changeup. Looking at Romo vs. right-handed batters, we can see a pretty typical usage for the slider. It’s generally in the lower half of the zone with most whiffs coming down and away from the batter. Against lefties, Romo has tended to work them away — or the “back door” slider — or down in the zone. I think from our graphs you can see the above-average control Romo has with his slider, he controls the pitch well and has tended to spot it in the right places.

More numbers on Romo’s slider:

* Of the 67 sliders Romo’s thrown this year, batters have whiffed on 18 of them. That’s a Whiff% of 26.8%.
* The league average Whiff% for all sliders (starters + relievers) thrown in the majors this year is 13.4%
* Romo posted a slider Whiff% of 14.2% in 2009
* The velocity range on Romo’s slider is between 77-79 mph

We’re looking at some pretty small sample sizes when it comes to pitches thrown and results, but Romo’s slider has been a fantastic pitch for him this year. It’s unlikely that he’ll continue to post a 26%+ Whiff% on his slider, but it’s seems likely that his slider will continue to be an above-average pitch for him. Whether or not he’ll have to add another wrinkle to his pitch selection as he progress through his career is another question that’s up in the air. Traditionally, Romo has thrown his slider almost half the time — 46% in ’09 and right around 40% this season — and batters have to know that when they are facing Romo, they’ll get a steady diet of sliders. And yet, they still have had trouble with the pitch. That makes the quality of his slider even more impressive, in my opinion.

As long as he’s healthy (my biggest concern with Romo) the Giants should have an above-average reliever on their hands. If Brian Wilson’s groin is still bothering him, you might even see Romo get a few save opportunities. For the pitcher with the funky motion and filthy slider, it’s well deserved.