For traffic to be effective it needs to be "targeted" i.e. people who are looking for your product or service right now when they're ready to make a buying decision, but how does this work? How do the companies supplying this traffic know its "targeted", well what they do is buy up lots of expired domains, these domains are no longer operated by the webmaster for various reason, they may have lost interest, they may have forgotten to renew their domain name (happens all the time), they are lots of reason, however these domain are still registered in the search engines and still receive substantial traffic.

Once the traffic companies buy the expired domains this traffic is redirected to your website, for example a person goes to Google and types in "cheap flights" and gets a list of results, they then click on the links in those result to find what they are looking for, if they click on a listing for a domain name that no longer exists but has been bought by the traffic company that click is then redirected to one of their client websites that relates to "cheap flights". From the surfers point of view this is seamless and they end up on a website relating to what they were searching for, the webmaster who bought the "targeted" traffic get a visitor who is interested in his products and the visitor find what he is looking for, so everyone wins.