The Rays traded a pretty good baseball player away in Matt Joyce, but the guy they got back is no slouch, either. Kevin Jepsen has pitched 280 major league innings over seven seasons, throws hard (fastball averages over 95 mph) and he's posted an acceptable 3.94 ERA that fails to match his very strong 3.16 FIP. That's a good number, but there's reason to expect better from Jepsen.
His 2014 performance was on a different level than what he's done in any other year. Normally, a relief pitcher coming off a season out of line with the rest of his career would and should be viewed with a healthy dose of small sample size salt (that's the pink kind), and that caveat still applies, but there was an obvious change in Jepsen's process that makes his improvement easier to believe. I'll let you find it.
Here is Jepsen's movement chart from 2013:
And now from 2014:
For 2014, Jepsen completely scrapped his cutter and introduced a changeup, which he threw a lot (most of those orange triangles at the bottom left side of the "changeup cluster" are actually miscategorized changeups as well. Here's the shift in pitch usage, visible in line graph form:
And here's an article about him introducing the pitch (h/t BossmanJunior333). It was something he threw when he was a starter, but scrapped in the minors when he moved to the bullpen.
It's obvious how changing his pitch mix helped Jepsen. Over the course of his career, the cutter had worked well for him, but it had lost effectiveness after the 2010 season. Here is a quick comparison of Jepsen's changeup to both career cutter numbers and the numbers from 2013. This data is from Brooks Baseball.
Without saying anything about pitch interactions, we can see that he replaced a less effective pitch with a more effective one. He was able to throw his changeup for a strike more often, and to produce more whiffs out of those strikes.
Maybe we shouldn't expect Jepsen to replicate his fantastic 2014 season exactly, but there's reason to believe that the improvement is real.