As of Thursday night, Billy Hamilton has been a top 20 player by Yahoo 5x5 since the All Star break. He's hitting .302 with a ridiculous 26 stolen bases, fueled by an on base percentage of .374, up from just a .283 OBP in games prior to the All Star break (ASB). The Reds have also slotted him in the leadoff position in the lineup, giving him more at bats per game and better clearance on the base paths. Hamilton is 26 for 28 in stolen bases in that time frame.
Digging a little bit deeper, Hamilton has increased his walk rate from 5.7% prior to the ASB to 10.3% post ASB. MLB average walk rate is about 8%, and Hamilton's career walk rate prior to the All Star break was 5.9%. According to research over at Baseball Prospectus, walk rate stabilizes around 120 PA for hitters, and Hamilton isn't far from that stabilization point with 107 PA since the ASB. There's a chance Hamilton has legitimately improved his ability to walk, although more PAs are needed to make any sort of long term conclusion.
He's also had a jump in BABIP from .293 to .367, which is not necessarily luck--although it can be. Better quality of contact, less pop ups or a better hit placement skill can help sustain higher BABIPs, especially for someone as fast as Hamilton.
One potential problem is that Hamilton is hitting .462 on ground balls since the All Star break, up from .340 prior to the ASB and .311 for his career. Sustaining such a high batting average on ground balls will be very difficult long term; in the PITCHf/x era, the highest BA on grounders by a qualified hitter is .363, by Odubel Herrera. Speedy generational hitter Mike Trout has hit .331 on grounders in his career. Hamilton will probably need to hit less fly balls to maintain that high BABIP; his 32% fly ball percentage post ASB is only a little under the league average of 34%. But nobody in baseball runs as fast as Hamilton, so that needs to be taken into consideration on ground balls as well.
It should be noted that there is danger in putting too much stock into arbitrary endpoints. Baseball is a random game, and weird things can happen over smaller samples. But players skill sets do improve over time, and if we as fantasy owners choose to wait for a large enough sample before taking action, those players are usually long gone on waivers by then.
Maybe Hamilton's increase in production lately is a sign of development in skill, or maybe it's just randomness and small sample size noise fueled by arbitrary endpoints. Maybe it's a little bit of both. But either way, it's something to keep on eye on going forward, because Hamilton is a plus on base percentage away from being a fantasy monster with his elite ability to steal bases, especially if the Reds keep batting him in the lead off slot.