Automatically motor driven loop will try to turn and stay at the direction which gives null signal or no signal. Operator would tune the receiver to the frequency of certain beacon, but we have simplified this and selection is based on beacon ID.
I'm pretty sure that all war time automatic D/F-equipment had a manual mode too because of meaconing. Meaconing will get the automatic mode confused. But we haven't planned any manual mode even simple meaconing is possible now.
Google is your friend:
http://en.wikipedia.org/wiki/Radio_direction_finder
http://en.wikipedia.org/wiki/Direction_finding
http://en.wikipedia.org/wiki/Radio_navigation
And the most epic navigation document ever which makes you appreciate the Art of Navigation.
http://aafcollection.info/items/docu...0198-01-00.pdf