That's not how it works. The idea is that the sites will hold your spot for X minutes before they release the seat, so what they're doing is booking the flight, choosing seats (or not), getting to the payment page, and sitting there while the timer counts down. I believe that once it gets to 5 minutes or so there's an alert that asks if you're still there, and if you say yes, they'll either reset the timer or add some additional time on.