Why the Line Moves
In the long run, sportsbooks make their money with the juice or ‘vig’ they charge for betting on each and every sporting event. When you choose a side against the spread and wager 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 a profitable model over time.
But because sportsbooks take action on both sides, they are almost always going against the betting public’s pick in terms of which one will end up being more invested. The exception being more money coming in on a particular side from a few big bettors opposed to more tickets from the public in general. This is sometimes referred to as ‘sharp’ money being wagered on a game that moves the line in one specific direction.
Sportsbooks Want Equal Action
Ideally for sportsbooks, each event or game would receive equal betting action on both sides. So if bettors were wagering a total of $11,000 to win $10,000 on the San Diego Chargers at -4 and $11,000 to win $10,000 on the Oakland Raiders at +4, regardless of which side wins, the sportsbook will still end up making a $1,000 profit.
The risk for sportsbooks occurs if bettors were wagering a total of $16,500 to win $15,000 on San Diego -4 and just $5,500 to win $5,000 on Oakland +4.0. If the Chargers win and cover the four points, the sportsbook stands to lose $9,500.
One-Way Action a Gamble for Books
The more lopsided the betting action is in one direction, the more sportsbooks stand to win or lose depending on the final result. For this reason, sportsbooks will often move lines in football and basketball in an attempt to lower risk and balance action.
For example, if the Philadelphia Eagles open at -6 against the Washington Redskins, and bettors love the favored Eagles and bet them hard at this price, the line will be moved to -6.5 to try to persuade some bettors to take the underdog Redskins. And if action continues to come in heavily on Philly, the line might move again to Washington +7. If bettors now prefer the Redskins at this bigger number, action will start to even out.
Keep in mind, the amount of betting action on an event or game will mostly determine which direction a line moves. How much or how quickly the line moves will vary between sportsbooks and depend on the amount of action a specific book is receiving on each side and how much risk the book is willing to take on a particular game.
Books that want more balance may move lines more quickly, and the most successful ones will always try to stay one step ahead of how they think bettors will react to their numbers.