How does Audirvana fold-down 5.1 mixes to Quad?

I’ve noticed when I play back 5.1 mixes from SACD .ISO in my 4.0 system, Audirvana seems to do a great job folding down those mixes from 5.1 to quad – can you point me to specifics on how Audirvana is folding down mixes, and are there any limitations to these fold-down algorithms?