One of the dumbest baseball articles I ever wrote was last year when I wrote that Luke Hughes was an improvement over Josh Donaldson. Hughes would play all of 4 games for the Athletics, and then be demoted to AAA, then AA, then released. He would finish the year with the Blue Jays AAA, and as far as I know, is out of baseball. Donaldson however, has emerged into a really nice player. He would finish 2012 with acceptable numbers, though still below average. In 2013, Donaldson has become a well above average player, even if you don't believe the defensive numbers. While BABIP has helped, his walk and strikeout numbers have legitimately improved.
So far this year, he is seeing a lower velocity than he was before 2013, 87.5 MPH versus the 2013 86.81 MPH on average. His swinging strikes are coming on harder pitches (86.96 MPH before 2013, 87.85 MPH in 2013), but there isn't a real difference in the average velocity of the pitches he is swinging at (87.58 MPH before 2013, 87.24 MPH in 2013). The average pitcher he is seeing, by release point, is about the same, though slightly shorter (5.88 v, -6 h before 2013, 5.83 v, -.57 h in 2013), and he is swinging at missing at pitchers that are more out right-handed than before this year (5.84 v, -.84 h before 2013, 5.84 v, -1.07 h in 2013). This would seem to be an improvement, as it is more likely that far out right-handers are the ones that are making him miss rather than more standard pitchers. However, he is swinging at pitches that are more likely to be shorter and further out right-handed before this year (5.89 v, -.6 h before 2013 and 5.84 v, -.66 h in 2013).
As far as location goes, here is Donaldson's average strike zone, broken down between 2013 and before 2013:
The big difference is the whiffs. Donaldson is now swinging through pitches further outside and further up than he had in the past. Donaldson is whiffing a lot less in 2013, going from a hitter who was whiffing more than average, to below average, becoming a better contact hitter (which is another reason the success is most likely real rather than a hot streak). Since the average pitch he swings at is no more or less horizontal, it suggests that a change in approach isn't helping him avoid swinging and missing, that he is just able to make more contact.
With that said, it does seem that the approach is better, not just because of his improved BB% and lessened O-Swing %, but because he is being pitched a little further out on average, and yet his average swing is staying the same horizontal, and on pitches more up in the zone as well. He is making pitches come up more, giving him a chance to drive the ball better.
Out of the 44 pitches he has seen at 95 MPH or more (as of the time I was writing this article), with 8 swinging strikes (18.2 %) with just 5 contact plays (11.4 %). Before 2013, he saw 68 pitches 95 MPH or over, whiffing at 14 of them (20.6 %) with 9 contact plays (13.2%). So he hasn't improved against plus fastballs, and that is probably the best way to get him out, so it seems hard to argue that his bat speed has improved. Instead, the average velocity data above shows that it is more likely he has improved on slow stuff. So I looked at the 100 slowest pitches he has seen in 2013 and before 2013. He is whiffing less at the slow pitches, almost all curveballs, (9 whiffs before 2013 and 6 whiffs in 2013), but is making slightly less contact as well (19 contact plays to 17 contact plays). This suggests that he just isn't swinging at as many slow pitches as he was (but also suggests that he is a good breaking ball hitter).
More than anything, it seems that Donaldson's skill set hasn't really changed, he is just being more discriminate at what pitches he is swinging at, making them come more inside on him. This seems to be why he has improved both in walks and power.