"There is a sound I heard occasionally in 'the old days' which I have not encountered lately. Though it is only rarely captured on recording, there can be evidence that it's present in the sound of a soprano. It is a glorious mezza voce: soft, delicate, floated - but it is also loud. It can soar over the full orchestra and above a chorus in full cry without losing its gentleness. I thought of it when listening to Tebaldi's recordings, know it was there with Milanov, heard it from Nilsson. Glorious and controlled as Caballe's mezza voce has been, that is not the sound I mean."
I think that to which you are referring is called by some the fourth period of Bel Canto, or good singing - Colour, Flexibility, The Messa da voce, Good Diction, and Accoustics. You will recognize it also in the Mozart recordings of Eleanor Steber and the early recordings of Luciano Pavarotti with Richard Bonynge. Ah yes, the messa da voce.
Many singers learn technique and learn the art of this fourth period of Bel Canto, but when it comes down to the actual performance, they wing it. Call it nerves, call it the show must go on, but all of the good work we do in the studio is lost. It is a tragedy and a sadness of heart. We Voice Teachers just go on. We try to instill and pass on the art form, but atlas, sometimes, OK, a lot of times it escapes.