Another one of those "it just works" technologies. My Soundbar is HDMI ARC capable as it my TV but if I hook it all up with HDMI it just loses sync with the picture. If I slip in the TOSLINK...perfect everytime.
You've never gotten the sender is sending dolby or dts and the receiver thinks it's PCM fun, I guess. Had that happen a few times. Doesn't sound very nice, but doesn't happen too often.