Index Betting - Sports Odds, Markets, Tips aka Spread Betting Index Betting is a form of gambling on the outcome of any event where the more accurate the gamble, the more is won while the less accurate, the more is lost. A bet is made against a spread (or index), on whether the outcome will be above or below the spread. The amount won or lost depends on the level of the index at the end of the event. The spread represents the index firms' margin.