One trick they used which I know well is booking one leg of the flight to a ghost or hidden city. For example my outbound flight was CLT>PHL>LGA. Not always the case but there are certain routes where it is cheaper to add a 2nd leg on. So in this case I booked that leg but just didn't take the flight from PHL>LGA. Incidentally this trick only works if you are carrying on your bag and you are on a one way flight. If you did this on a round trip on your outbound flight the airline will cancel the rest of your ticket.
What impressed me though is that they found a flight that I could not find on the airline's website. I don't know if it was a code share flight with some other airline but I looked all over USair's site for the flight I bought from some random discount site and could not find it.
I have a bit of experience with this. Flight "hackers" use a number of varying techniques. As simple as using low-cost airlines that are not registered with the main services, to very crafty means to circumvent fuel surcharges. travel and flight hacking is a very interesting world!
