Hidden city ticketing is when you purchase a ticket with an extra segment at the end, but you never intend to go to that final city. For example, if you are in City A and you want to fly to City B, sometimes it’s less expensive to purchase a ticket A to B to C. In this case C is a third city that you do not want to visit.
Some of us select less-popular departure times (e.g., 6 AM, 11 PM) because those tickets cost the least. Wouldn’t it be nice if the airline called you and said they had a more desirable flight time available, maybe with fewer stops, and you could switch to it at no extra cost?
Well, that is an option when American or United does a schedule change.
This article has the details, and includes the different rules that Delta follows. For American Airlines, you want a schedule change of 90 minutes or more. For United Airlines, you want an arrival time shift of at least 60 minutes.