Legend has it that Satchel Paige was so confident in his ability to strike out batters that he would call his outfielders to the infield, inviting them to take a seat and bear witness to his greatness. You probably won't see Yu Darvish motion for Nelson Cruz any time soon, but you couldn't blame the Texas Rangers ace if he did.
Darvish has whiffed 72 hitters in 45.2 frames this season, a staggering 14.2 per nine innings pitched. His closest competitor among qualified starting pitchers is Max Scherzer, with a comparatively puny 12.4 K/9. Yu is punching out batters at a pace that even Pedro Martinez and Randy Johnson couldn't manage in their primes, and he's doing it by throwing nearly twice as many sliders as he did last season.
As a rookie, Darvish threw his slider about 19% of the time. Batters swung and missed 44% of the time against the pitch, a mark bested only by Zack Greinke and Edwin Jackson among starters who threw at least 500 sliders. This year, Darvish has thrown his slider 37%. Hitters aren't catching on, though, whiffing at an even higher clip (46%).
Darvish has finished off 43 of the 72 batters he has struck out (60%) with a slider. The increase in sliders thrown is a major reason why Darvish's opponent contact rate has plummeted, particularly to the glove side:
Darvish's contact rate by pitch location, 2012
Darvish's contact rate by pitch location, 2013
Yu has only made seven starts, but it's worth noting that no starting pitcher in history has ever punched out hitters at the clip he's currently managing over the course of a full season. Darvish's current K/9, if it were to hold up, would put Pedro and The Big Unit to shame. Kinda makes you wonder if the Rangers need outfielders on the day Darvish pitches.
Highest single-season K/9 for qualified starters