For example, suppose you're flying to a conference that takes place mid-week, but have to buy tickets that include a Saturday night stay. So you need to pick a Saturday night, either the one before the conference or the one after. On the night before you want to leave, call the airline and tell them you'd like to return early. For example, if you want to fly out on Friday, call them Thursday night. This isn't flying standby, because they'll actually give you a reservation for the earlier return.
Some travel agents use a similar trick to obtain inexpensive tickets for their business customers.
They'll issue a ticket with a Saturday night stay at the discount rate, and provide the passenger with a sticker to put on the return ticket after the outbound ticket has been removed. The sticker reflects the earlier return flight. The travel agent also changes the return date on the reservation after the outbound ticket has been used. Since this is a slightly shady practice, any other changes in the itinerary must also be made through the travel agent, not the airline. Not all travel agents are willing to do this, and then only for expensive tickets and their best customers.