it is a hub and spoke. They will fly you home on American to your base at the end of your reserve trip always (unless you magically appear there a few hours earlier thanks to JetBlue and so many others we have done so many times).

The reason you would be stuck in Chicago for an extra 2 or 3 days after a 2 day trip is because they are really short on pilots in Chicago, so they usually just reassign you until you reach your last day of reserve, then deadhead you home on American to your base (Boston in his senario).