This statistic looks at the rotation and the defense as a unit. The rotation's line drive rate, for instance, and other factors will affect how difficult it will be for the defense to convert a BIP. Most of the time this stat is listed along with the DER per pitcher.
Maine has a good swing for a pitcher but on anything that moves, he has no chance. And if it's a fastball, it has to be up in the zone. Basically, the pitcher has to hit his bat. - Mike Pelfrey