Spread betting is used to create an equal or active market for all sport's participants in one bet, even when the outcome of an event may appear already to be biased towards one of the competitors in a certain bet.
Between sports, spread betting is used at Basketball betting mostly but can be also used at Hockey, Tennis, Volleyball and most of the other sports where the bookmaker can intent to create an equal market.
In a sport's event, a traditionally stronger team may be playing against a much weaker team where the fixed odds are completely pointing out that stronger team is going to win, most of the sport's matches have a favorite and an underdog.
If the bet is "Who will win" or "Will the favorite win", in most of the cases most bets are to be made on the favorite. However, a spread is the range of possible outcomes and the bet is whether the outcome will be above or below the spread, not the simple question whether one team will win or not.
Because this method has the intention to create an equal number of bets on both sides, the probability to win these bets is mathematically equal to 50%.
The point spread is essentially a handicap towards the underdog so the bet becomes the question: "Will the favorite win by more than the point spread?" or "Will the favorite cover the points spread?".
The bookmaker advertises a spread of 9.5 points in a certain game:
Los Angeles - San Antonio (+9.5)
Home Odds - 1.91, Away Odds- 1.91
* You bet 100$ on Los Angeles to win -9.5 points at 1.91 odds. To win your bet, you need Los Angeles to win the game with more than ten points difference. If they do, you win 100$ X 1.91 = 191$. Your profit is 191$ – 100 = 91$.
Most online bookmakers use half of a point difference, just to avoid possible draws where money returns. If Los Angeles fails to win the game with ten points or more, no matter even if they win with less than ten, your bet is lost.
* On the other hand, if your bet was San Antonio to win with +9.5 points at 1.91 odds, to win your bet, you need San Antonio not to lose the game with more than nine points, no matter if they have actually won the game, or lost it with nine or fewer points difference.
As said, spreads are mostly used as half-point fractions to eliminate the possibility of a draw (also known as tied or pushed bet).
A teaser bet is a variation of spread betting, with an option that allows the bettor to alter the spread in his favor by an already determined margin. For example, if the originally set odds were Los Angeles -9.5, the bettor can alter the handicap at either side he wants.
If the bettor is confident in a huge win for Los Angeles, he can alter the handicap to -15.5 and get much bigger odds than even odds are and also on the other side, he can alter the handicap to less than 9.5 and get odds less than even odds are.
In both cases, he is changing the predicted possibility to one of the sides, but at the same time, he is changing the odds to either of sides.