When an active domain name expires, it could still be renewed by its present owner for about eight weeks after the expiration and despite the fact that it won't be active, it can't be obtained by anyone else. If the domain is not renewed by then, however, it gets into a 5-day deletion period and after that it's available to the public. During the deletion period anybody can submit a backorder for the domain through an ICANN-licensed registrar and the service provider will use its registry channel in order to try to register the domain for the user immediately after it is available. As quite a few registrars can attempt to acquire a given domain and names are acquired on first-come, first-serve basis, there's no guarantee that employing such a computerized service will grant you ownership over it, still this is the most efficient way to get an appealing domain name as trying to register it manually almost certainly means that you'll be way too late.