Yep I want to know as well. I think they offer link positions in high pr websites, how they get them I don't know.
About 422 of them here: http://danabbamont.wordpress.com/dofollow-backlinks-guide/ It's just blog comments, forum post, user profiles. They just post relevant stuff and put links wherever they can. I mined Angelas and there were thousands but I filtered the list down to the dofollow ones and sorted them by PR.
Angela and Paul sell backlink subscriptions at the warrior forum. Angela charges $5/month for 30 links and Paul has a couple packages available with $8 for 50 links being the cheapest. Every month they send a new set of links. The homepages have PR 4 - 9 but the page the links go on is generally PR0.
Yes, the work well but there are people who base their linking in just these profiles sites so Google penalize their sites soon. As Vesuvio says they can help you for little competitive search terms but don't over do it.
Usually these links are from profile links but all are PR-0, clear mess. You don't need to waste ur time to build PR-0 backlinks. These sort of links won't work for long term.