Arbitrage betting ("arbing" or a "sure bet") means backing every outcome of a market at different sportsbooks so the combined implied probability is under 100% — locking in a guaranteed profit whatever the result.
Different books price the same game differently. When one book is generous on side A and another on side B, the sum of the two implied probabilities can drop below 100%. Staking both sides in the right proportion returns the same amount either way — for more than you put in.
Arbs are small (1–5%) and vanish in seconds to minutes as books adjust. Books also limit or ban accounts that arb soft lines. Speed, wide book coverage, and using exchanges or crypto books is how people actually run it.
In most places it's legal, but it violates most sportsbooks' terms and can get your account limited. It's not risk-free in practice — books void bets and change limits.
By scanning many books at once for markets under 100%. Doing it by hand is too slow; an odds API or scanner surfaces them in real time.