The policy is included in Southwest's Contract of Carriage (pdf download). Accommodation for delayed flights is spelled out in multiple places, but significantly as shown below: 4. Tickets a. Tickets (4) Delays or Involuntary Cancellations. If a Passenger’s scheduled transportation is canceled, terminated, or delayed before the Passenger has reached his/her final destination as a result of a flight cancellation, Carrier-caused missed connection, flight delay, or omission of a scheduled stop, Carrier will either transport the Passenger at no additional charge on another of Carrier’s flights, refund the fare for the unused transportation in accordance with the form of payment utilized for the Ticket, or provide a credit for such amount toward the purchase of future travel. With regards to a delay allowing standby on an earlier flight, I've found that this is very much up to the discretion of the agent, including the necessary length of delay to trigger. Also, it seems this is allowed less now following the change to allow A-Listers free anytime standby.
... View more