korn, I believe you answered your own question, as yes there are definitely games where players are allowed to "get off" should they choose to, and others where they must open/expose themselves up if wanting to do so, yet also games where the books are willing to take plenty more..usually if the game sits just right and/or they want the action?
There appear to be many variables involved, ranging from original line, current score, betting handle(smarts,public,volume), anticipated 2nd halve betting(sharp, public, volume), indicators/telltales of current gameflow, not exposing selves to Polish-middle, being sided,etc..
If I'm not mistaken, Richard Klamian used to help set some halftime numbers?
Yet, it could be argued that BB sets the NFL halftime numbers quite often!