Essentially they approach gas stations in a cluster and say "Hey, who wants to offer our users a discount?" They reach an agreement with one of them and advertise it - "Get six cents a gallon off at this one Exxon." Then they go to another cluster, etc. Their hook is that only one gas station in a cluster of stations is part of their deal -- but it also means that there's usually one station wherever you go that has a discount.

Of course, you have to be careful in that sometimes the discount and the starting price still don't match the starting price with no discount of another place, but I've found that you can usually find something along your way somewhere that provides you with a discount worth stopping for.

If you want, I can send you my code so you'll get an extra discount your first time through. Just let me know.