Find the Hidden Technique

For my singers and opera aficionados:

See if you can identify the secret technique which these master singers and teachers only hint at.
Look for it
– in performance: Placido Domingo and Luciana Serra – (check out the clips where it’s not hidden by costume or camera angle)
– and in teaching: Montserrat Caballe, Birgit Nilson, Renata Tebaldi, Elizabeth Schwarzkopf