He has elite velocity (98.0 MPH sinker) and an elite secondary pitch (slider). It is elite because it has 10.3 inches horizontal break. As you might expect, it gets a high rate of whiffs (.159 whiffs per pitch to right-handed batters and .139 whiffs per pitch to left-handed batters).
In December, the Diamondback signed him because he was the type of player they needed in the bullpen, and with the expectation that he could improve. Three things that Mike Hazen mentioned were better use of his sinker, increased use of his slider to left-handed batters, and more consistent location of his pitches.