An acca is a group of individual bets brought together in one mega wager. Each leg of your acca, or accumulator, must win in order for you to win the overall bet and get a payout.
The method for finding arbitrage opportunities entails looking for significantly differing odds on the same sporting event. If the odds differ greatly enough, there is a reasonable chance for arbitrage. A betting calculator will tell you how much opportunity is available. It helps to look at some real-world examples.
