Opening Vegas college betting lines are established as a 'point spread' that the book believes to attract exactly half of the action on each side. In determining the opening line, the book largely considers the record and strength of schedule of the two clubs, game venue, injuries and other important factors. The betting line may change in either direction when more money is played on one team.
For example, consider a college basketball game between the Michigan State Spartans and Duke Blue Devils where the Blue Devils are seven-point favorites. The betting line would appear as Duke -7 or Michigan State +7. If a larger portion of money was being played on the Blue Devils, the line may shift to Duke -7.5, -8 or -8.5 to attract more bets on Michigan State. Likewise, if more money was coming in on Michigan State, the line might shift to Michigan State +6.5, +6 or +5.5, to draw more bets on the Blue Devils.
The Vegas books take a small commission on each wager, known as 'juice.' Therefore, by adjusting the spread to encourage equal money played on each team, the book is guaranteed to profit on each contest it accepts action on, while avoiding the heavy loss.