The initial line by the books is just a kind of feeling out process. They set the line kind of where they think it should be based upon a variety of factors, including past experience that a lot of fans will bet the Patriots, for instance, or bet a favorite. Then, as the sharp money comes in they adjust it accordingly.
The sharp money waits for the initial line, waiting to pounce. They usually have their own "line" in mind, and if the initial line at the books is different enough, they will hit it hard.
Sometimes they'll put some early money down on one side just to see if they can get a shift in the line from the books, then bet big on the other side later in the week. A lot of sports books will have lower betting limits early, then raise them as the week goes on.
It's kind of interesting to watch. You might see a game where the line is set, and then a few days later you'll see that 85% of all the tickets are on one side but the line hasn't moved. That tells you that the public is the 85%, but the sharp money is going against the public and is putting enough money down to balance out the large disparity in tickets sold.
