This is a very common occurrence throughout the sports betting industry. Sportsbooks have the right to shift the spread or odds for any given match prior to it starting. Many factors play a huge role in this decision, and they include injuries, weather, the volume of bets on one side, and anything in between. Depending on the time you place your wager, the bettor may also have an advantage or disadvantage based on which way the spread has shifted.
Say the Bears are a six-point favorite over the Lions and the total is 42. To bet the Bears, you must "lay the points," meaning they must win by seven or more to cover and give you the win. Betting the underdog Lions, you are "taking" six points, and they can lose by five or fewer, or win the game outright, and you have a winning bet. If the Bears win by exactly six, both sides "push" and all bets are returned. It's also a push if the final score equals 42, otherwise the over or under will win.
For beginners, the moneyline bet on an underdog is a great choice as it will allow bettors to win 50% of the bets and still earn a profit. Each of these bets have benefits and drawbacks. With moneylines, punter shave the appeal of betting on a winning side. This is a great choice for die-hard fans. Many people find it more appealing to bet on a winner of a game instead of betting on the end score of the game. Spread betting does offer some nice benefits. They are simple yes or no bets. Either the team covers the spread or they don’t. However, moneyline bets typically offer the chance to win more than is bet, so these are often the choice for many bettors online.
Shopping betting lines is one of the most important things you can do when betting point spreads in basketball. While the majority of sportsbooks will have the same line, it's fairly common that you can find lines that are a half or full point different. This can have a huge impact on your bottom line. If you don't think a half point or one full point makes that big of a difference, just ask anyone who has bet sports for a while. They will inform you that getting an extra half point is like winning the sports betting lottery.
Identify the favorite: Lines with a - before the number (i.e. -200) indicate the favorite. A -200 should be read as: "For every $200 wagered, I win $100." When there is a negative sign, the line should always be read with relation to 100. That does not mean you have to bet that much, it's just easiest to understand! When a + sign is present, just reverse the reading, always keeping reference to 100:
The optimal situation for bookmakers is to set odds that will attract an equal amount of money on both sides, thus limiting their exposure to any one particular result. To further explain, consider two people make a bet on each side of a game without a bookmaker. Each risks $110, meaning there is $220 to be won. The winner of that bet will receive all $220. However, if he had made that $110 bet through a bookmaker he would have only won $100 because of the vig. In a perfect world if all bookmaker action was balanced, they would be guaranteed a nice profit because of the vig.

Here you can see that the Rams are +3.5, while the Cowboys are -3.5.  So for this example the Cowboys are 3.5 point favorites, while the Rams are underdogs of 3.5 points.  If you were to bet on St Louis you would need them to lose by 3 or fewer points or just win the game outright.  If you were to bet on Dallas you would need the Cowboys to win by 4 or more points.

To show how this can be exploited, take a point spread of -8.5 at odds of -110. This is a 50/50 proposition. Let's assume you've purchased enough points to move the spread to -6.5 at odds of -150. Now, you'd win instead of lose 6.24% of the time they win by 8, and 6.59% of the time they win by 7. Add these together with the 50% from the original proposition, and we get 62.83%.
Oddsmakers want you to gamble on underdogs as well as favorites. They set points spreads that encourage balanced betting. They analyze the strengths and weaknesses of each team, factoring in won-loss record, strength of schedule, results against common foes, key injuries, recent performance and previous games between the teams. They also rate the value of home field advantage and consider the game day weather forecast where relevant. If they see heavy wagering on the favorite, they will increase the point spread during the week to spur more betting on the underdog. If more money is going on the underdog, the spread will decrease as game time nears.

Oftentimes you’ll see a point spread that has a half-point added to the number. Of course, there’s no such thing as half a point in a football game, so why do we so often see point spreads with a (.5) attached to the score? Sportsbooks do this to make sure there isn’t a chance of a push.  Let’s take another look at our game from above with the half point added.