Chris Capuano, the Mets’ new left-handed pitcher, has a strange platoon split. Against right-handed batters he gets grounders like Jered Weaver (37%), but against left-handed batters he gets them like Felix Hernandez (54%). The average pitcher has a fairly large difference in strikeout and walk rates by batter-handedness (Capuano’s strikeout and walk splits are standard for a lefty), but a small difference in ground-ball rate. Dave Cameron found that left-handed pitchers get only marginally more grounders against left-handed batters (46%) than against right-handed batters (44%). Capuano’s ground-ball split is out of the ordinary.
So where does it come from? Capuano is a three-pitch pitcher with a sub-90s fastball, a change and a slider. He throws the slider more to left-handed hitters and the change more to right-handed hitters — like most left-handed pitchers. But that isn’t the root of the difference because his change up to right-handed hitters gets more grounders than his slider does to left-handed ones. Rather, the difference is all in his fastball which gets 60% grounders against left-handed hitters versus just 35% against right-handed hitters. (This is for Capuano’s 39 appearances covered by the Pitchf/x data, 15 of his starts in 2007 and all of his appearances in 2010).
So what is going on with his fastball? Unfortunately, the Pitchf/x data does not yield an easy answer. He locates his fastball slightly farther outside against left-handed batters than he does to right-handed ones, but most left-handed pitchers do this. There is no real difference in the height of his fastball by batter-handedness, and the release point of his fastballs against the two groups is also pretty much the same. The answer to this strange split with his fastball might be deep in the pitchf/x data, but it would take some serious digging. Do Brewers fans have any insight?
Through his career Capuano’s main weakness has been the long ball, and it looks like this has been mostly from his fastball to right-handed batters. The move to spacious Citi Field will likely mitigate some of this weakness. But Citi Field reduces home runs to left-handed batters more than right-handed batters, which isn’t as much of a help to Capuano who gets tons of grounders from left-handed batters.
Edit: Garik16 over at BtBS has taken a look at this question question. Check it out.