Chris Volstad (FLA) has done well throwing the changeup this season. He's second in opponent's batting average (.063) behind Ryan Madson's change (.057). Last year, opponents hit .246 against his changeup, putting Volstad in the bottom half of the league.
Here's a look at how his changeup this season compares to last.
Volstad has lost a little vertical movement (BrkZ) on his changeup, while gaining some horizontal movement (BrkX).
Batters have swung less and chased fewer of Volstad's changeups this season. Yet, they have swung and missed slightly more and failed to produce more than one hit. Perhaps the increase in left to right movement on the pitch has been more effective against batters, even at the expense of less downward movement.
I'd hesitate to suggest he will be able to sustain this success. Batters still make a great deal of contact against his changeup. Volstad has struck out only two batters with it this season. However, he's only thrown the change 13 times with two strikes. Given that a BABIP correction is more than likely forthcoming, he'll need to start throwing the change more when batters are down in the count.