Basically, the way it works is that the average single is worth ~0.5 runs. That’s the additional runs scored you’d expect to get in an average inning that has a single vs. that same inning before the single. So, if you have a guy with excellent range in the outfield who prevents that single, you prevent that 0.5 runs. But in addition to preventing the single, you also caused an out! That extra out reduces the number of runs scored in an inning by, on average, 0.3 runs, because it prevents other players from coming up to hit as Krall describes above…