One of the underrated ones was "spin it." As in, "He can spin it," meaning throw a quality breaking ball. I remember a Gardenhire monologue after a lefty specialist named Randy Flores was brought in and threw fast balls to a left-handed hitter, who got a crucial hit. Gardenhire's rant was to the effect that if he wanted fast balls thrown to that guy, he'd have left the righty in, and Flores needed to "spin it." (As I recall, Flores never got the ball again in a game situtation.)
Well ... with the advent of Statcast, we now have objective data on pitch spin. As a general rule, the higher the spin rate, the better -- spin creates friction with the air and thus makes the pitch move, and more friction means more movement.
And Ryan Pressly has impressive spin rates. Rhett Bollinger of MLB,com posted this piece earlier this week noting that Pressly's average spin rate ranks eighth among MLB pitchers (he didn't say what the minimum innings or pitches for that ranking would be).
That suggests that Pressly has the talent to be a successful reliever. So far, he hasn't really had the results -- a 3.70 ERA isn't what you expect from a short man with dominating stuff.
Bollinger says Pressly expects to increase the use of his slider this year, probably by decreasing his curve. It may make sense for him to pick one breaking ball and stick with it; few pitchers throw both the curve and the slider with proficency. Pressly himself cites command glitches as a problem, and I suspect Neil Allen, the pitching coach, would agree.
Basic point: Pressly isn't an outstanding reliever now. But he has the stuff to be one. And this is a bullpen that certainly has room for somebody to emerge.