They set the opening line. They are a group that use several different formulas including their own power rankings, and handicap the game. Once they set a line & the books put it on the board, it moves almost immediately by sharps, which are LARGE money bettors. If sharps pound a line at -5, it will move quickly to -5.5, then maybe to 6, & so on until they can start taking in decent amounts on the +. Some college basketball totals, for instance, may move 10-12 points. I have seen many times opening lines at bookmaker move 6-7 points on totals in 30 seconds after being released