Joe Blanton gets very different reactions to his fast and off-speed pitches. This graph shows Joe's results by different speeds in 2010.
Batters like to swing at the off-speed pitches, but their contact rate goes way down. Less contact, means more swing and misses. Blanton gets a 30.7% strikeout rate on the soft stuff, 8.5% on his fastball. So while batters hit .345 when they put a Blanton fastball in play versus .383 on the softstuff, the higher strikeout rate means opponets hit just .261 on the off-speed pitches but .313 on the fastball. That makes Blanton's slow stuff all or nothing pitches.