RDM stands for Remote Device Management.
If you have a RDM compliant console, interface and fixtures it is possible for the software to interrogate the fixture and get it's DMX channels etc and then set the DMX address according to what is required by the software. No more fiddling around with dip-switches or menus on fixtures to set the address. Very handy when you go to a venue and you want to control the venues lighting as you can retrieve all of the fixtures DMX addresses and have them made available at the console.
This is being done over the normal 3-pins (not using the extra 2-pins on a 5-pin cable/connectors), in half duplux mode, I am sure this is because there are so many 3-pin devices and cables it would be fool hardy to try enforce a 5-pin standard and because certain lighting companies have adopted the other 2-pins for their own use, which on some of them it is power.
It does not constantly send/receive data it is put into what they call discovery mode to get info back from RDM compliant fixtures. Each fixture will have a 48-bit UID, a bit like a MAC address on network cards etc., it will keep sending out this “discovery” message and when it gets nothing back it will know that it has found all the fixtures.
As for how this information is then processed and used is down to the software that companies will write.
Apparently existing DMX splitters will not work with RDM because of it's bi-directional nature.
It also allows for retrieval of run hours, lamp hours and other information etc. if that is available from the RDM compliant fixture.
This is still in it's infancy and will take years before all fixtures, interfaces and consoles (hard or soft) will be able to use this feature fully.