Last Saturday during the Astros-Angels game one of the announcers said something about how Bregman has become more disciplined this year and knows the strike zone better, so that has helped him hit better with RISP (he is 5th in the league or MLB, can't recall which).
I asked on the Yahoo Statistical Analysis group if there are any studies that show that guys with more plate discipline do better with RISP, either in general or relative to what they normally hit.
D.K. Willardson responded and did a study, looking at the relationship between OSwing % and RISP AVG minus Overall AVG (OSwing % is the percentage of pitches outside the strike zone that batters swing at). D.K. looked at Qualified Hitters 2015-2017.
The r-squared was .0372, meaning that only 3.72% of the variation in Overall AVG minus RISP AVG is explained by OSwing %. So guys who are more disciplined do improve with RISP, but not much.