The ratio of Z-Swing% to O-Swing% shows a pretty good correlation to FIP, where the lower the ratio, the greater the success. Among 201 starters with at least 30 innings pitched so far this year, Doug Fister's ratio of 1.82 is the lowest in baseball.
I'm not saying this is going to continue. If you asked me to make a list of the most deceptive pitchers in the league, Doug Fister wouldn't make the cut. But a big part of his success so far has been hitters going after junk while taking a passive approach to balls in the zone. Though many of his strikes seem hittable to the naked eye, they're only hittable if you swing, and over five starts, batters haven't swung.
If and when Fister's ERA returns to Earth, I think I know how it's going to happen.