When an existing domain expires, it could still be renewed by its current owner for about sixty days after the expiration and even though it won't be active, it can't be taken by someone else. In case the domain is not renewed by that point, however, it gets into a 5-day deletion time frame and then it is available to the public. During the deletion period anybody can submit a backorder for the domain through an ICANN-accredited domain registrar and the company will use its registry channel in order to attempt to register the domain name for the user the instant it's available. As a lot of registrars may attempt to get a certain domain and names are acquired on first-come, first-serve basis, there isn't any warranty that employing such a computerized service will give you possession over it, still this is the easiest way to get an appealing domain because trying to register it personally almost certainly means that you'll be way too late.