Sean O'Sullivan (KCA), right-handed pitcher, found a way to fool left-handed batters this season. In 2010 they hit .247 against him with some power, slugging .428. This season, lefties managed just a .177 BA against Sean, with a .226 slugging percentage, as a home run is the only extra base hit they collected.
What is O'Sullivan doing differently? One thing that stands out is his increased walk rate against lefties. While the BA against dropped 70 points, the OBP against him only dropped 16 points. Sean traded walks for extra base hits. While a high walk rate isn't a good idea overall, for Sean so far, discretion is the better part of valor.
O'Sullivan also changed the movement of his pitches. He throws a hard sinker and a slider (although the sinker doesn't seem to sink that much). The following chart shows the movement of those two pitches against left-handed batters:
The hard sinker (the large red area above the horizontal line) tended to move away from lefties. The slider moved down and in. In 2011, the movement of both pitches shifted in:
So a pitch that used to tail away from lefties now tails in on them. The slider moves farther in on them than before. So a batter used to reaching for one of these pitches is now going to find the ball hit off the handle instead of the sweet spot. It strikes me that this improvement might not last. Once LHB get used to this movement, they should be able to hit it as well as last season. Eventually, O'Sullivan may want to mix in both sinkers, so batters can't prepare for one or the other.