I believe that it allows PCM5.1 to be re-encoded as AC3.I dont get that. I dont have ffdshow as post-processing and i still get ac3, dts, ... passthrough.
What does it add?
This is needed for 5.1 AAC and 5.1 WMV over SPDIF.
Hoborg may have other reasons too, but without post processing you would only get 2 Ch SPDIF for those two formats