basically it is the use of a buffer site in between to prevent google from detecting a link exchange. there are plenty of ways to build backlinks for free so there isn't even any need for link exchnages anymore. lik exchanges take more time than one way link building and takes a lot of time in management as well. oftent a site you are exchanging links with will go dead, some will remove your link after getting alink back so its hard to keep track. one way link building is the way