Hard work, very good teachers, both good dancers AND professors (not all are !)
if you want to see what modern tango is, nothing to see with gomina, machos y putas :
http://fr.youtube.com/watch?v=OTpKzlX9qO0


P.S. of course, even after more than 6 years of studies I'm totally unable, and will never be, to do half what they do.