Well, they don't spend much time at all talking about the nuts and bolts of the stat, apart from saying that not only does it model the likely effect of lots of variables for every pitch (who is hitting, who is fielding, hopefully who the umpire is, what ballpark are you playing in, etc) but it updates the values of each of those variables as time goes by. IMO, it is interesting and is something that is overdue to be explored, but it sounds like over-fitting is a real concern https://en.wikipedia.org/wiki/Overfitting, and I want to know more details about what variables are being used...and how. I've always wondered if a stat that compensated for game situation would be useful. For instance: should a walk with the bases empty and no outs with a two-run lead in the 8th be recorded as the same result as a walk with a man on second in a tie game and 1 out in the 5th? How much more valuable is a HR with no one on, down 2 in the 9th than a walk in the same situation...or a single? Is a walk actually the best outcome for a hitter, since they didn't have to risk putting a ball in play for an out? How much can we credit the pitcher or hitter with for a particular outcome before we are adding more noise than we started with?