To begin, lets first look at Maness' pitch usage over the last two seasons, as that will give us and idea of what to examine in a more detailed fashion:
As you can see, Maness is really a two pitch pitcher. He uses his sinker the vast majority of the time and then offsets that with his changeup.
First, lets take a look at his sinker usage and location:
Unsurprisingly, Maness works the lower middle of the zone quite frequently with his sinker, but one key to his effectiveness with the pitch is that he rarely misses the strike zone with it. He's not pounding down around the knees with the pitch and missing low a lot. One reason that he can pitch so effectively in the strike zone is the inability of hitters to do much of anything with the pitch other than slap it on the ground at a very talented Cardinal infield:
As you might expect, this diminishes the ISO on the sinker to below-league-average levels:
This is the real secret of Maness' success, honestly: He can throw his sinker into areas of the strike zone that hitters normally crush the ball for a high ISO and get ground balls. This ability allows him to keep his walk rate at a minuscule level (just 4.5% of PA's against Maness end in a base on balls) so that he doesn't hurt himself by putting runners on. It's also one reason why Matheny is so confident in using him in situations where there are runners on base (or at least I hope that's the reason, rather than the whole "double play dude" thing): He doesn't walk anyone and rarely gives up an extra base hit.
So how is Maness so successful at keeping hitters from driving his pitches? One reason is movement:
The horizontal movement on the sinker is nearly 8 inches from the point of release to the time the ball reaches the plate. The negative sign indicates that the pitch moves to the left of center, or towards the Maness' arm side. That, coupled with 3 inches of vertical movement shows you why hitters have a tough time getting the barrel on the ball against him -- basically the pitch is moving in on the handle (RHH) or out towards the end of the bat (LHH) as it moves toward home plate, creating a lot of weak contact.
You can see that his changeup also has a significant amount of movement as well in a similar direction as the sinker. Why is it so effective? Well, look at the velocity for one thing, it's almost exactly 6 mph slower than the sinker, which, if you believe the theory of Effective Velocity (or EV for short) is about the perfect gap in velocity for pitches thrown in the same location. The other reason for the changeup's effectiveness is where it's thrown:
Maness is painting the lower arm side quadrant with his change-up, with a good number being strikes. In contrast to the sinker, which is thrown over the plate a great deal, the change will start in the lower half of the arm side and basically run further that direction, all at 5-7 mph slower than the sinker. That makes it a very effective pitch not only at creating weaker contact, but also at keeping hitters off balance at being able to time Maness' best pitch, the sinker. Considering that it's rarely ever thrown when Maness is behind in the count, you could also consider it a bit of a "put away" pitch.
Just these two pitches alone can create a lot of havoc for hitters, especially since they will only see him for one or two PA's at a time. But Maness can also mix in a few other offerings to really screw up hitters timing and approach. Case in point: The slider and curveball.
Breaking pitches are rarely ever thrown to left handed hitters:
Even to right handed batters, Maness only throws his breaking pitches when he's ahead in the count (0-1, 0-2, 1-1, 1-2, 2-2 counts account for > 90% of all breaking pitches thrown). Not only that, check out where he throws them:
Translation: The curveball and slider are out pitches for Maness. He only throws them in pitcher's counts and only down in the zone off the outside corner. Most of these are sliders, as well, which makes sense: When you live in the zone with sinkers and changeups that run in on the hands of right handed batters a great way to get a swing and miss is to throw a pitch when you're ahead that breaks hard the other way and out of the strike zone. While the whiff rate isn't high on those pitches, nearly all of them that are put in play turn into ground balls:
The rest are nearly all whiffs where hitters are swinging over top of them. These two pitches aren't successful by themselves and certainly aren't plus or even average-plus, but by setting them up with sinkers and changeups running the other way and then picking the perfect count to throw a pitch moving the other direction.
Now, one would think that a pitcher that just pounds the bottom of the zone would tend to struggle more as hitters adjust and begin looking for the ball in that lower half that they can square up. That's where the four seam fastball comes in. It is more of a "show me" pitch that he used to change the eye-level of hitters when they're sitting on his sinker in the bottom half of the zone:
He rarely throws it to left handed hitters, much like his breaking pitches, and it's generally not thrown for a strike either and is almost always thrown in a 2 strike count. Many of these are "waste" pitches, where Maness is trying to force the hitter to respect the high part of the strike zone by throwing a ball up and away and immediately coming back with a sinker (or, more likely, a changeup) that's moving down and in to get the hitter to swing over the top of it.
Seth Maness is a master of command and pitchability, which allows him to get away with having only one plus pitch and a myriad of other fringy offerings that he has total and full command of. Keeping hitters off balance is the key to his success, and while he might struggle when facing a lineup 2 or 3 times through, as he would in the rotation, it's easy to see why he is so effective in short stretches out of the bullpen.