Bronson Arroyo has certainly put up some outstanding numbers in the second half of the season. In his first 17 starts he posted a 5.85 ERA, which wasn't far off from the 6.08 FIP that he put up over the same span. That is to say, he sucked and a lot it was his doing. In the 15 starts since then though, he's posted a sparkling 2.15 ERA in large part because he has cut down his non-intentional walks and his home runs allowed in half. Those two improvements alone have removed over a run and a half from his ERA, but that is not all that has given Arroyo more success in the 2nd half.
While Arroyo's ERA is hovering around 2 in the second half, his FIP is still just 3.80, good but not great. A lot of that difference can be attributed to the help that Arroyo has received from his defense. Check out the break down in this table:
As you can see, outside of pop flies, Arroyo hasn't really changed the rates of the batted ball types that much, but his defense is turning a lot more of those batted balls into outs (DER calculates the rates that balls in play are turned into outs). Couple this with the fact that Arroyo has dramatically cut down on his walks and you can see why he has faced much fewer base runners since the all star break. Add in to this a lower home run per flyball rate and you have the makings of a Goodroyo transformation. This all despite the fact that his strikeout rate has only increased slightly and is still well below his previous rates with the Reds.
To give you an idea of how much the fewer base runners has helped, in his first 17 starts, 10 of the 21 HR that Arroyo allowed were with runners on base, and 4 of those were with 2 outs. In his last 15 starts, just 1 of the 10 HR that he has allowed have come with runners on base. Being able to get outs faster has meant avoiding the big inning, which was Arroyo's doom in the first half.
What this all says to me is that Arroyo is likely do for a tumble, not that you wouldn't expect that. His HR/FB rate is probably a bit low and his walk-rate is a few ticks below his career rate. If those settle back to their norm without any improvement to his strikeout rate, we'll start to see him wonder back over the 4.00 ERA range rather than the 2.00 ERA range. Also, while the defense was below their norm behind Arroyo in the first half, they've been much more above the norm in the second half. If he starts getting more normal defensive help (the team's seasonal DER is .707), you'll see a few more longer innings with a couple more base runners, leaving Arroyo to fend for himself to get the outs. Sometimes that's not a good thing, though having Arroyo at his 2007-2008 levels still makes him a valuable pitcher. He just won't likely be as good as we've seen this second half.
NOTE: All of the data that I used for this was compiled from Baseball Reference, who combined regular fly balls with infield flyballs in the reports that I ran. I separated out pop flies using hit data from Pitch FX, which may not be completely accurate. I still think the point is accurate, even if the numbers aren't exactly what you would expect.