These are popular with casino and affiliates both. When a visitor leaves your site without clicking through to your sponsors they are presented with a pop under loaded behind your sites window. This shows a member of the exit exchanges sites. You will get a corresponding pop under for your site when the same happens on another members site. The usual ratio is 2-1, meaning you will have to show 2 sites to have your site shown once. You can find a 3-2 ratio at times but don't settle for less. Since you may be new and have no real traffic to get views of your page you will want to go back a section and read it again in case you missed the answer. Understand, that you are not the only person doing this so the general pool of traffic will be of less quality then one may expect but it is still a better then totally untargeted traffic. Probably one of the largest and oldest in this area is:
As with all traffic you buy, make sure you know who you are dealing with and check them out. Some casino exit exchanges are reported to shave you closer then a 3 blade razor.