[I'd like to point out this entire article - in particular the penultimate paragraph - was written and scheduled before last night's game, where Schoeneweis was kept in to face a right-hander and allowed the go-ahead three-run homer. However, it is now even more appropriate, since judging by that fiasco, it seems AJ Hinch might benefit from a refresher course in some of the basic concepts. Bitter? Hell, yes. Wouldn't you be after your team's bullpen allows ten runs in one inning? Anyway...]
We've all seen it happen. The opposition's star left-handed batter is at the plate, and the manager strides out of the dugout, gesturing with his left-hand to the bullpen. In comes our LOOGY - Left-handed One Out Guy, the southpaw specialist whose career is based on situations like this. Sometimes it works - see Todd Helton vs. Scott Schoeneweis last weekend. Sometimes it doesn't, as on Opening Day, when Seth Smith of the Rockies was the first batter Schoeneweis faced, and went yard.
Left-handers are a good deal more common on the mound than among the general population. Studies estimate that, while the numbers are higher among the young, only around 12% of 20-year olds are left-handed, and this isn't far off the percentage of position players who throw left-handed, which is 13.7%. However, this season, 168 of the 570 men to have delivered a pitch are lefties - that's 29.5%, more than twice the rate among hitters. [As an aside, the number of position players who bat left-handed is much higher: 31.3%, with another 15.9% being switch-hitters. I'm not sure whether this was a conscious decision by them to change; I know that personally, I always threw left and batted right, it just seemed the most 'natural' way]
Generally, batters perform better against pitchers of the opposite hand: righties like lefties, and vice-versa. It's easier for batters to see the ball when it's thrown from the other side, as the delivery point will be closer to the center of their field of vision. In addition, breaking balls to a RHB from an LHP are more likely to move in towards him, which is easier to hit. As a concrete example, just think of Randy Johnson's slider, slashing down and away from a left-handed batter, and the wild swings and misses it generated. That's one of the main reasons they hit .199 lifetime off the Big Unit, and why so many southpaw sluggers were 'rested' with Johnson on the mound.
With over 70% of pitchers - and more than 72% of plate-appearances this year - coming agianst right-handed pitching, you'd expect the additional experience to help batters. Yet curiously, since 2005, right-handed pitchers have been fractionally more successful than lefties overall. as measured by OPS. Over the past four full seasons, the difference in their favor has ranged from zero to twenty-one points of OPS, averaging out at 7.5 better than their sinister brothers. But this conceals the platoon splits based on the hitters they face. Let's break the 2009 numbers down by the handedness of both the pitcher and the batter:
The difference, around 54 points of OPS, is at the heart of what managers are trying to mine with regard to managing the match-ups, and seems to have remained relatively constant over the years [.I checked the 1979 stats and the differences were 61 and 65 points] The offensive team wants batters to face opposite-handed pitchers; the defense want them to face same-handed pitchers. The method by which this is accomplished has changed over the years, with the focus moving from platooning position players, to using the bullpen, because the balance of a typical roster has changed.
Gone are the days when a team could go through an entire season using only ten pitchers - last year, the number varied between 18 and 32. Now, most teams carry 12 pitchers, only four or five bench players, which reduces the chance to platoon them regularly based on pitching match-ups. While starting lineups may be influenced by whether the opposing starter is left- or right-handed, we see a lot less these days of the wholesale platooning, whereby one player will only start against RHP, while another gets all the LHP starts.
A manager also needs to take into account the individual batter. For example: you're playing Seattle and the winning run is on second, with Ichiro at the plate. As a left-handed hitter, you might be tempted to reach for your LOOGY and get the OPS benefit. But if you look at Suzuki's career splits, you'll see something odd: he hits southpaws better than right-handed pitching - instead of being -55, he's +35 points of OPS. On the other hand, this could still be countered with a particularly lethal LOOGY. Schoeneweis might fall into this category: his career OPS vs. LHB is an amazing 237 points lower than RHB, at below .600. That's why gameday threads start to froth whenever he's kept in to face a righty. 'Skins has a diagram, I believe.
The use of the left-handed relief specialist is one reason why it is generally recommended to split up your left-handed batters when constructing the line-up. If you have a large 'lump' of such hitters consecutively, it's easy for the opposing manager to pop his LOOGY in, and get him to face multiple batters with the platoon split working in his favor. If the line-up alternates, he either has to burn another pitcher, or lose the advantage. All these factors should be taken into consideration when deciding on what pitcher to use for a particular match-up. It's almost like chess, with managers thinking about the potential moves the other can make, bringing in a pitcher to counter a pinch-hitter, or the other way round, and looking several spots ahead in the order. However, hopefully this primer will have given you some idea of what's involved in the process.