A friend's status on Facebook brought these Utube selections below to light, that is, Joan Sutherland and Marilyn Horne talking technique with Will Crutchfield at The Juilliard School  in 1998.  

I was there, sitting in the balcony.  It was quite something to see the two of them in action.  And I remember these two vocal giants not quite agreeing about how to breath when singing: Dame Joan was an ' abdomen must go in' gal, while Horne was a 'you must push your diaphragm out' person.  Whatever one makes of that, I am ever thankful to Dame Joan for one thing: it was from her that I learned about Herman Klein and his connection to Manuel Garcia. That bit of knowledge opened up a whole world.

Breath Support and Open Throat

High Notes and Passaggio

Chest Voice and Messa di Voce

Note May 2, 2018: Sadly, the links for this post are no longer working and have been removed which has as much to say about the nature of Youtube as it does the need for real libraries—where information doesn't disappear when accounts are closed and lights go out.