When a domain name expires, there's a time period of a little over sixty days when it's inactive, but could still be renewed by its current owner. During this time period, the domain cannot be taken by anybody else, but if it isn't renewed, it gets into the so-called Pending Deletion period which lasts five days. This is when ICANN-accredited registrar firms take backorders for such domains. When a specific name is backordered, the registrar will make an attempt to register it for the customer right after it's deleted and accessible to the public, which gives people a chance to obtain a short and easy to remember domain that may have been used by someone else until now. In case the domain is rather appealing, a number of companies can try to register it in the same time and due to the fact that this happens on a first-come, first-serve basis, there's no guarantee that a particular person will get that domain name. Nevertheless, using a backorder service will give you a lot better chance to obtain a specific domain than if you attempt to register it manually [
Source].
The power of a backorder service depends on the number of registars they own. Multiple registars allow them to increase the number of queries and therefore increase the win rate, beating the competition.
You can understand why DropCatch is the service that captures the most disputed domains if you take a look at this list:
https://features.icann.org/compliance/registrars-list