Rather than being a plate-appearance-by-plate-appearance statistical simulation using real players' statistics, like Strat-O-Matic, Brett Ball is a pitch-by-pitch tactical simulation using archetypal players for each team's lineup.
Each team's lineup consists of an identical selection of 9 batters falling into one of five broad categories: high average, low power, extra speed; high average, high power; decent average, decent power; low average, low power; and (optional) pitcher. There's also a selection of pinch hitters, one from each category (except pitcher), and a pinch runner.
The player on defense starts the action by rolling a pair of dice; if the total is between 2 and 5, it's a 'hittable' strike (the lower the number, the easier it is to hit); 6 or 8 (except 4-4) are foul strikes; 7 is an unhittable strike; 8 (4-4 only), 9, and 10 are hittable balls; and 11 and 12 are unhittable balls. If it's a hittable pitch, the offensive player decides whether to swing; if they do, they roll the pair of dice, and consult a chart to determine the results. Base runners can attempts steals and (sometimes) advancing an extra base on a hit; the defensive player can call pitch-outs if a runner is on base; and the batter can be made to hit-and-run, sacrifice bunt, or even suicide squeeze. The usual rules of baseball apply (3 outs to an inning, 9 innings to a game, etc.).