When the url expires it enters in to a "Registrar hold" or grace period for 30-45 days so the current owner has a chance to claim it. If the current owner has not claimed the url name after the initial 30-45 days grace period it enters in to a 30 day "redemption period" then if it still has not been renewed it goes into Deletion Status for about 5 days the gets dropped.
once it is in the process of being dropped, depending on how popular that domain name is, there is a possibility that it may have been backordered as well, so your chances of picking the domain name up are very minimal.