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Swinging strike % (SwStr%) is the percentage of pitches thrown that result in a swing and miss.  It's a quick way to tell how good a pitcher's "stuff" is and correlates strongly with strikeout ratio.  For some context, average SwStr% for relief pitchers is around 9.5% and Juan Cruz led the National League at 15.4% last season.  Bobby Parnell's % is an oddity, as he's generating more strikeouts than expected.  He has also yet to give up a home run and unfortunately that will change at some point.  If J.J. Putz has more outings like he did yesterday (94 mph average fastball, 95.8 max fastball), his SwStr% should rise.  (stats via Statcorner)