The odds are constantly updated, in many cases more frequently than even the most popular paid membership sites. Sports Betting Arbitrage, Scalping, Sure Bets, and Arb Trading all refer to the one thing, the idea of making a guaranteed profit from a difference in odds between sportsbooks. Normally, backing all outcomes of a single sporting event at a single bookmaker would result in you guaranteeing a loss of a few percent - this is the bookmaker's margin.
The Free Odds Comparison Service