As part of a discussion in the Facebook group to answer a question regarding this, Cristina posted this useful paper: http://web.tiscali.it/lpweb/rel_cacuminali_poro.pdf As well as the Wikipedia pages on the sounds. https://en.wikipedia.org/wiki/Voiced_retroflex_stop https://it.wikipedia.org/wiki/Occlusiva_retroflessa_sonora I would suggest if you have questions regarding this distinctive sound of Sicilian (and apparently other languages of the area) I definitely suggest taking a look at this paper!
