One of the biggest decisions the Chicago Cubs made during the winter was exercising their $20 million club option on left-handed starter Cole Hamels. Acquired at the deadline from the Texas Rangers, Hamels posted a 2.36 ERA in 12 starts with the Cubs while showing no signs of his early-season home-run woes. Still, it wasn't clear that the Cubs would retain Hamels -- as our Matt Snyder wrote at the time, the option was "a lot of salary for a pitcher heading to his age-35 season, especially if the impulse to pick up the option is based on just 12 starts."

More than a month into the regular season, the Cubs seemed to have chosen well.

Through six starts, Hamels has a 3.19 ERA (138 ERA+) and a 3.08 strikeout-to-walk ratio that would represent his highest since 2015. He's leading the Cubs in innings per start, with more than six, and is tied with two other veteran southpaws (Jose Quintana and Jon Lester) for the most quality starts on the staff. He's been better than the Cubs could have reasonably expected.

What makes Hamels' season most notable, however, might be his emergence as an elite groundball pitcher. So far this season, he's coerced grounders on 58 percent of his batted balls, the highest rate among all qualified starters. For reference, Hamels had previously topped 50 percent just once -- and that came in 2011, when he generated 52 percent grounders.

Typically, when a pitcher's batted-ball profile is altered like this, it stems from a difference in approach. Hamels has made tweaks, but probably not the one most would anticipate. Rather than throwing more sinkers this year, he's throwing fewer. Instead, Hamels has leaned more on his four-seam fastball, while maintaining his usage of his secondaries -- his trademark changeup, his cutter, and his curveball. Each pitch is generating more grounders than in the past, with the change resulting in nearly 80 percent groundballs, while the curve (70 percent) and cutter (50 percent) are also during their fair share of worm-killing, per Statcast.

Perhaps the key to Hamels' dirt-covered success is rooted in location. He's throwing his four-seamer further down in the zone than in recent seasons, according to Brooks Baseball. As such, then, he may be creating a different interplay between pitches that, in turns, has batters topping the ball more often.

Whatever the exact reason for Hamels' groundball infusion, the Cubs have to be happy with the results -- and with their investment.