The stat issues points for postive accomplishments like retiring a batter or recording a strikeout, and deducts points for negatives like walks and hitsallowed.
Pitchers were primarily rated on winning percentage, earned run average (ERA) and innings, with limited weight given to strikeouts and (inversely) walks and hitsallowed per nine innings.
The last 10% of his score was comprised of how he stacked up against the league average in strikeouts per nine innings, fewest walks allowed per nine innings, and fewest hitsallowed per nine innings (3.3% apiece).