Arbitrage can mean buying a package of items and selling another equivalent package of items. So in horse racing (vague 20 year old memories here), it might mea

Arbitrage can mean buying a package of items and selling another equivalent package of items. So in horse racing (vague 20 year old memories here), it might mean betting on the a certain proportion of your money on the trifecta and other proportions on each of the horses not included in that trifecta. So you'd have do the math for each combination of 3 horses that could appear in the trifecta, and the corresponding payoffs. I doubt he had something that placed the bets automatically (because betting wouldn't have been on the internet) but he had some software to take the odds and identify arbitrage opportunities.