I won't spend time looking at his delivery because I already did that in my breakdown of the Lions' deliveries.While his head is straight at the time of release, it appears that there is some arm drag as his body is closer to the plate than a pitcher normally is by the time he lets go of the ball. Because of this, his posture isn't that great, which could cause him to jerk the ball. In his actual delivery, he uses a large leg kick and hesitation to get deception, and hides the ball with a very slight body twist after the pause.
Before trying to take a look at what exactly went wrong with Wakui, first let's take a look at what Wakui is currently. Below is a quick summation and look at his pitches. Each GIF, excluding the change/fork GIF, is from a pitch this season. All 2013 pitch data comes from Michael Westbay and ScoutDragon, while all data 2012 and before comes from NPB Tracker.
Wakui's slider averaged about 80.4, a full MPH tick below what it was in 2011-2012.
His curve is a slow curve, averaging about 69.7 MPH, and thrown about 7.3 percent of the time
The data says that Wakui throws both a change and a fork, but for our purposes here, I combined them. He threw the changeup just over six percent of the time in 2013, and the forkball just over 5 percent of the time. The changeup is about three kilometers slower, at about 79 MPH.
Now that we have taken a look at what Wakui is at this point and what to expect from him, I think looking at why he regressed from dependable starter to mediocre reliever. Anecdotally, it appears that the slider has lost a lot of sharp downward movement, but things like movement from year to year can be really hard to quantify without Pitch F/X data, so I thought I would look at location. First, here is a breakdown of Wakui located all his pitches in 2011, his last full year as a starter, where he had a 99 ERA - and 4.32 kwERA (he had actually seen a drop off of strikeouts that year, signally the start of his regression). Each square is broken down by percent, and it is from the pitcher's perspective, so the far right would be the arm side part of the plate for Wakui:
If Wakui was going to throw a pitch outside of the zone, it was going to be low and glove side, like a slider. His most common location overall is glove side and middle, though if he was going to go high in the zone, it was more likely to be arm side. This isn't too unusual, as it is traditional fastball/slider locations for a right-hander. Compare this to his 2012 graph:
As a reliever, Wakui increased his fastball usage, which helps explain why there is an increase in middle to high arm side pitches. However, instead of throwing the changeup less as a reliever, he threw it more in 2012, subtracting his slider usage more, which helps explain the difference in low and glove side pitches (a disappearance almost). However, it is tough to see him increasing low arm side pitches. It appears that overall, he was working higher in the zone more across the board. How about in 2013?
Obviously there is a huge difference in pitches thrown low and glove side out of the zone, with a heavy increase. There are less pitches high and arm side as well, going for more glove side pitches in general. While he saw a slight bump in slider usage in 2013 compared to 2012, but it was still lower than it was in 2011.