General,
The theory is that the line before the cincy game starts would be a weighted average of the line assuming Denver uses all its starters and the line assuming they rest some, where the weights are the winning probabilities of the 2 teams in the cincy game. If betting were allowed on denver during the cincy game, then the line would look like an interactive betting line, but since that requires a lot of attention on the part of the bookies, most choose to just take it off until the game is over. If cincy had won the line would probably have moved the other way a bit.
I'm not sure if this is how it actually works but if it did work this way there would be no arbitrage opportunities. And since it is very important for bookies not to allow arbitrage opportunities I am assuming it actually does work this way.