In the long run, sportsbooks make their money with the juice they charge on each game. When you choose a side against the spread and bet 11 to win 10, the book is essentially making a bet of 10 to win 11 on the other side. You can see how that would be profitable in the long term.
But because sportsbooks accept action from both sides, they are at the betting public’s mercy as to which side they will end up being more invested.
As an example, in a perfect world (for the sportsbooks), each game would receive equal betting action on both sides. If bettors were wagering a total of $11,000 to win $10,000 on San Diego -4.0, and $11,000 to win $10,000 on Oakland +4.0, regardless of which side wins, the sportsbook will win $1000.
But what if betters were wagering a total of $16,500 to win $15,000 on San Diego -4.0 and just $5,500 to win $5,000 on Oakland +4.0? In this scenario, if San Diego covers the four points, the sportsbook stands to lose $9,500.
The more lopsided the action is in one direction, the more the sportsbook stands to win or lose depending on the decision. It is for this reason that sportsbooks move lines in football and basketball; to attempt to lower risk and balance action.
For example, if Philadelphia opens at -6.0 against Washington (+6.5) and bettors love Philadelphia and bet them hard at this price, the line will be moved to -6.5 to try to persuade some bettors to take Washington. If action continues to come in heavily on Philadelphia, the line might move again to Washington +7.0. If bettors now prefer Washington at this number, action will start to even out.
Betting action almost always determines how a line moves. How much or how quickly a line moves will vary from book to book, depending on the amount of action their specific book is receiving on each side and how much risk they are willing to take on a particular game. Books that want more balance may move lines more quickly.