From Paterva Wiki
This transform queries Robtex's database to determine which networks have their reverse DNS delegated to this IP address. This is a very useful transform in the infrastructure foot printing process. Keep in mind that the IP address needs to that of a name server (that handles the reverse zones). In many cases this transforms provides better information than simply looking at routing or whois information. This is because organizations might have a full class B network but are only using three or four class C networks within the bigger block. In many of these cases they will only have reverse DNS information populated for these smaller blocks - and you can find these smaller blocks using this transform. See  here (section on Reverse Delegation) for more information.