H-20 No Stereo on Optical Output - DirecTV DVR

For H-20 No Stereo on Optical Output, relating to DirecTV DVRs and receivers. I've been following a thread on another forum about not having stereo on the optical output unless there are RCA plugs connected to the RCA audio output jacks on the back of the receiver. This then turns on the stereo for the optical output. The plugs in the RCA jacks don't have to be terminated to anything, just inserted. There is a third contact in the receiver RCA jacks thats acting as a switch.
Can anyone explain why this is so? Or is it a design flaw.