I agree with mzpro5. there is no noticeable difference in sound using a optical cable or a coax cable digital cable. both send the digital signal from point A to point B.

some people prefer the coax because it provides a better connection on both ends. An optical cable by compairison seems very unstable and may become loose over time.

As for the "switching" term.. there again is what mzpro said. You connect all your audio and video connections from your SAT, CD, DVD, etc. into the rear of the 990 pre amp. then the 990 pre amp actions like a "switcher" routing the correct audio and video signal for you. It's a bit more complex than that.. but that's the basic idea behind having a processor that does the switching.