Your logic above sound correct, when your 6.1/ES is set to "Auto."
Although Yamaha and Kenwood use different terminology, here's what I think is happening to you (I know it is to me). When the 6.1/ES is set to "Auto" then it only sends a rear center signal when the media actually HAS a discrete sixth channel encoded. In other words, it sends 5.1 when it gets 5.1, but 6.1 only when it gets 6.1
On the other hand, when you turn the "Auto" Off and enable the decoder with your remote, the Yam will send a matrixed signal to the rear center when a discrete signal is not available. So in this case you always get a rear center channel signal, even from 5.1 source media.
(My Kenwood does the same by toggling from "ES Auto" to "ES (always) On" with my THX switch.)
BTW, it is often not "faking" the rear center: on many media produced prior to the "discrete" sixth channel standards, the information for a sixth rear channel was already being encoded into a 5.1 standard using the same matrix technology that your receiver can then use to extract it again. The matrixing is simply older technology.
Your receiver might still generate sound to all six (in your case eight) speakers from a stereo source such as a CD. It all depends on what listening mode you set on the receiver.
I don't know whether you can create a "ES matrix decoder" signal from that though, you'd have to check your manual. Theoretically, the ES matrix decoder is taking the left and right surround signals (DTS or Dolby Digital) to generate the center. But if your receiver will turn stereo into multi-channel (so that the surrounds are getting some kind of signals), then I suppose the ES could then make another channel out of that... You got me on that one.
