Digging deeper into Hammel's slider, we can see why it has been especially effective this season. According to movement data from Brooks Baseball, Hammel's slider has had its most downward vertical movement this season. Given that his slider, as evidenced in his outing against the Marlins, is more dependent on vertical break than horizontal break, this is a very good development.
Overall, Hammel's slider has an 18.8-percent swinging-strike rate. Isolating for only sliders that opposing hitters have offered at, his whiff rate jumps to 41.7 percent, which is the ninth highest in the majors according to Baseball Prospectus. Below is a table with results and averages hitters have amassed against each of Hammel's pitches this year. Again, we see the effectiveness of the slider, here, as hitters have managed just a .152 batting average and a .217 slugging percentage.
The next table shows what sort of batted-ball outcome occurs when a hitter swings at Hammel's slider. In addition to whiffing 41.7 percent of the time, they foul it off 24.4 percent of the time. That means they put it in play only about one-third of the time that they swing at it. As the table shows, rarely do they do so with any sort of authority.
Hammel's ground-ball rate on his slider is just shy of 50 percent, and his popup rate is nearly nine percent. Hitters have managed a 19.3-percent line-drive rate against the slider, but that translates to about 12 percent of swings. Any pitcher can live with that total. But even with all the data above, perhaps the best illustration of how good Hammel's slider has been is the final chart below, which is a heat map of the where in or out of the zone he has thrown the pitch this year.
At the same time, Hammel's .234 BABIP suggests he has been a bit fortunate this season. His overall line-drive rate is 20.2 percent, while his ground-ball rate is down at 39.9 percent. The slider has been quite effective, but his other offerings have left a bit to be desired. Should the Cubs deal Hammel to an American League team, his stock would certainly take a hit by virtue of losing a handful of pitcher plate appearances each game. Even if he remains in the NL, it's hard to imagine him keeping up this level of production. A 31-year-old pitcher rarely changes his stripes.