Jeez. It seems like the Rays start an ace on the mound every game. One of them is Matt Garza who is widely believed to have the best "stuff" on the Rays staff and is the most likely one to throw a no-hitter or a perfect game. This season is looking like it will be the best so far in his career going 5-1 in his first six starts with a 2.09 ERA and 3.28 FIP. His best off-speed, the slider, continues to work for him as it has since he joined the Rays in the 2008 season. According to MLBAM pitch f/x data, Garza's slider has got contact rate of about 60%. That is ninth best in the Majors for a pitcher who threw the slider at least 70 times. And out of that same data set, Garza is third in run values per 100 pitches.
Stealing the idea from Dave Allen, here is a plot of all his sliders this season marked by pitch outcome.
Notice how most of his sliders are down and away to righties and down and in to lefties. Garza has thrown the slider in the wide pitch f/x strike zone only 32.7% which is the lowest among the data set mentioned earlier. Although that shouldn't stay there long since the lowest last year was about 33.7%.
Now using the pitch f/x data from the past three seasons, we can try to see where Garza will most likely get a swinging strike based on location and batter handedness. First up, right-handers.
Remember, this is just whiff rate. All swinging strikes divided by all pitches. So looking at that large red orange blob just under the strike zone, we see that Garza's whiff rate around that area is about 40-50% with a smaller blip about half a foot away. Now let's look at the same plot except for left-handers.
Note the legend difference in this plot with that small red cluster specifying a whiff rate of almost 100%! Garza threw the slider about a third as much to left-handers than right-handers so the regression may be little misleading. But there is a whole lot of green which is about 40-60% just below the strike zone, which is good because that is where you want the slider to be most of the time.
The slider isn't what Garza solely relies on for success, but looking at the run value leader board from last season, there was a lot of Cy Young winners, Cy Young vote getters, and potential Cy Young award winners that had the slider as their best pitch.
Data from Fan Graphs and MLBAM.