On the last day of June, American Airlines agents at Gainesville Regional Airport in Florida canceled a North Carolina teenager’s flight after realizing that he had used a booking tactic called “skiplagging” or “hidden-city travel,” forbidden by airlines but used by some travelers to net flight bargains.

The teen’s father, Hunter Parsons, said it was his 17 year old’s first time flying unaccompanied and that both the timing and price of the booking made it appealing — for $150, his son was to fly from Gainesville to Charlotte, and not continue on the second flight to New York, a cost savings of roughly $300 if the family had simply purchased a direct flight to Charlotte. But Mr. Parsons’ son didn’t even make it past the check-in counter in Gainesville, where the airline agents questioned why the teen would fly to New York when his layover was in his hometown, Charlotte. He was forced to pay for a direct flight.

Skiplagging is buying a ticket for a connecting flight, with a layover in the city that’s the real destination for the traveler. Flyers disembark after the first leg and simply fail to board the second. Often the fare is cheaper than if they’d actually bought a direct flight to their desired destination. At least two websites now help unearth these deals for consumers.

Though it is not illegal, the practice is strictly prohibited by airlines in their contracts of carriage. And carriers have shown an erratic but heavy hand in administering punishment for those caught, eliminating a skiplagger’s frequent flier miles in one instance and suing a passenger in another. Mr. Parsons said that American banned his son from traveling with the carrier for three years.

Experts say that travelers have flown this way for decades and that detecting improper use of connections is tricky, making enforcement a challenge for airlines. Here’s what to know about this open secret of flying.

How does it work?

Airline ticket costs are determined by many factors, such as energy prices, passenger demand for certain routes and competition from other airlines. They aren’t merely a reflection of travel distance. For travelers, this means that it can occasionally be more affordable to book a flight with a layover along a more competitive route and get off there, rather than booking a nonstop flight to the desired location.

For instance, a traveler in Fort Lauderdale, Fla., wants to fly to San Francisco. A recent search turned up a flight from Fort Lauderdale to Portland, Ore., with a stop in San Francisco, for about $124. A nonstop flight from Fort Lauderdale to San Francisco would cost about $220.

Skiplagged and Kiwi facilitate searching for and booking these kinds of deals.

But it requires staying undetected. You can’t check a bag or link a frequent flier account to collect the mileage. And travelers who show patterns — flying the same routes repeatedly and missing the last leg of a flight — are more likely to be caught.

Why is it gaining in popularity?

Short answer: the cost of airfare, which has soared in recent years.

Mary Cropper, a travel specialist with Boston-based Audley Travel, called airfare pricing a “perfect storm.” Rising fares, in addition to fees for everything from beverages to printing boarding passes, have made skiplagging more enticing to passengers. While she doesn’t advise her clients to book this way, she said she understood the motivation.

“Why wouldn’t someone want to save 50 percent on an airline ticket, something that’s just so expensive these days?” Ms. Cropper asked.

Aktarer Zaman, Skiplagged’s founder and chief executive, emphasized that his website has “empowered people to make their own decisions” by digging up alternatives to current airline pricing practices, which he described as “price gouging.”

What are the risks?

Upon purchasing a ticket, travelers agree to a contract of carriage with the airline, which is a set of rules passengers agree to comply with. These cover everything from how a carrier will handle oversold flights to requirements regarding passengers’ clothing and behavior. These terms and conditions also often explicitly forbid hidden-city ticketing and lay out a raft of consequences should travelers circumvent the rules.

United Airlines reserves the right to permanently ban travelers who don’t complete their trip. They also say they can revoke status and wipe out accrued mileage. Alaska Airlines mentions that it can take legal action against passengers who violate their policies.

American says in its fine print that it has the right to a number of actions, such as canceling any unused portion of the ticket, refusing to let the passenger fly and billing the customer for what the intended route would have cost.

Curtis Blessing, a spokesman for American, called this type of travel an “all-around bad outcome” that could cause issues with checked bags and also prevent other customers, who might have urgent travel needs, from booking seats.

Airlines have mostly taken aim at the online providers, in past years filing a handful of lawsuits against Skiplagged and Kiwi. Earlier this summer, American sued Kiwi in federal court in Texas, claiming that the “rogue online travel agent” had repeatedly engaged in “abusive ticketing practices,” including booking hidden city fares for its customers, according to court documents.

So far, litigation has been unsuccessful in blocking this travel hack. One lawsuit filed in 2014 by United against Skiplagged was dismissed, while in 2021, Southwest Airlines sued Skiplagged for using the carrier’s data. Skiplagged argued that they were not the ones actually distributing Southwest tickets; they showed flights but directed users to other websites for booking. Both parties later agreed to drop the lawsuit, Mr. Zaman said.

In 2018, Lufthansa sued a passenger who had booked a round-trip flight from Oslo to Seattle but then stayed in Frankfurt, the flight’s layover city. The airline later dropped the lawsuit.

However, there are plentiful anecdotes online of passengers who say they’ve been threatened with lifetime bans and with the loss of all their frequent flier mileage.

Is it an ethical way to travel? How much of an issue is it for airlines anyway?

“The question is whether you think it has any impact on the provider and whether you care,” said Robert Mann, an aviation analyst and a former American executive. Airlines say that exploitation of hidden-city fares results in empty seats that could have been booked by others.

In its lawsuit against Kiwi, American also said that passenger no-shows can delay flights by triggering searches for the traveler and any luggage involved, and waste fuel that is calculated by passenger count.

Others say that these fares are a loophole of an airline’s own making, and a reflection of their pricing priorities. By law, airlines can set whatever prices and routes they want.

“It can be argued charging much higher prices for shorter distances is unfair and especially penalizes frequent fliers to hub cities,” where high demand generally means costlier airfare, said Paul Hudson, the founder of a passengers rights advocacy group called Flyers Rights.

What can you do if you’re penalized for skiplagging?

Very little. Bart Banino, an attorney at Condon & Forsyth, a firm specializing in aviation law, emphasized that passengers are obligated to adhere to the airline’s terms and conditions — that contract of carriage — whether that means traveling on the route they’ve purchased, or paying a fare difference when they’re caught breaking the rules.