When an existing domain expires, it can still be renewed by its present owner for about two months after the expiration and despite the fact that it won't be active, it cannot be obtained by anybody else. If the domain is not renewed by that point, however, it enters a 5-day deletion period and after that it's released to the public. Throughout the deletion period everyone will be able to submit a backorder for the domain through an ICANN-approved domain registrar and the company will use its registry channel in order to attempt to register the domain name for the user once it's available. Because lots of registrars may try to get a particular domain and names are acquired on first-come, first-serve basis, there is no guarantee that using such a computerized service will give you ownership over it, still this is the best way to get an attractive domain because trying to register it yourself almost certainly means that you'll be way too late.