Okay, I've now wasted 17 minutes looking at intense data on Detwiler's fastball, and I am mega-impressed. He has two fastballs: the regular and the sinker. The sinker is #1 in baseball this year for movement. It was #1 in baseball last year for movement. Detwiler is #1 in baseball this year at getting people to swing (Cliff Lee's #2).
The thing with his two kinds of fastball is, they're released from the exact same point. They're at exactly the same speed. But one of them drops 5 inches and moves 3-4 inches horizontally, and the other has a foot of "rise" and a foot of horizontal movement. With identical release points and identical velocity, there's no way to tell them apart. As a hitter, you're guessing as wildly as hitters were guessing when they faced last year's #1 swings leader...R.A. Dickey.
Detwiler's gotten lucky so far, since he doesn't strike many guys out and has left 90% of his runners on base. Eventually that will change and those runners will start scoring. But even so, his sinking fastball is one of (FanGraphs says) the 10 most effective pitches in baseball right now.