Yu Darvish is an incredible pitcher. We've already seen him pitch a near-perfect game against the Astros, and we've also seen him make a pretty outstanding transition from Japanese League baseball to the MLB.
One of the keys to Darvish's success, as well as every other great pitcher, is the ability to fool the batter by throwing a variety of different pitches from identical arm angles and release points.
By making every pitch look nearly identical, pitchers make it much harder for the batter to tell which pitch is coming. When you're forced to guess which pitch is coming, and you have less than a half-second to figure it out, it makes it awful difficult to hit.
Combine that with the ridiculous movement on some of Darvish's pitches, and you have a nearly impossible task.
The internet geniuses over at Reddit compiled this incredible animated GIF of 5 Darvish pitches at once. Notice the identical release point, and then the 5 completely different places where each of the pitches finally reach the plate.