My experience is very simple. I think that my greatest guru was to try and to find right positions, right sounds, right resonances, every day.
This is normal.
But exercises without theory is useless. To explain the opera singing technique to my students I want to show them the sonograms and power spectrum of their sounds. In Italy there are a lot of very famous (?) teachers, but they don't know the meaning of formant, or harmonic, and they explain using conventional-without-sense words only.
In my opinion there is 1 technique for each overtone singer, for each opera singer, and so on.
There are some common rules, but our vocal tracts are different, so everyone try to develop his own feelings and positions.
We know that to develop our ability we must work with:
* larynx position
* soft palate position
* tongue position
* lower jaw position
* lips position
Ok, for my sounds there are two very important elements:
* my tongue position
* my lips position
Many people thinks that the tip of the tongue is extremely important. For me it can be important, but I think to the root of my tongue, for me that is really important. The tongue root movement involves the larynx movements, and they work togheter to develop formants, expecially the High Singing Formant.
If I try to move up and down the tip of my tongue I can obtain little sound differences about specific overtones, but if I move up-down the tip of my tongue and forward-backward all my tongue, I can develop different overtones.
This is my experience, this is in MY vocal tract, maybe in your too.
Then there is a great confusion about nasal cavity and nasal resonance. It is impossible produce overtones only by nasal resonances, because formants develop themselves into the vocal tract. Nasal cavity is an appendix of our vocal system. We know that air into the nasal cavity develops a counter-formant, a "valley" between two formants.
So it is very important that we must work to try the correct percentage between nasal resonances and vocal tract resonances. This is very important for opera singers too. When an opera singer finds his correct positions, we say that he has found the sound "in maschera", in other words in the face pack. The sensation is to feel a little tingling, like a group of little insects in the nose, lips and cheeks.
Generally the correct movement of larynx must be not like a yawn, it is useless, because the vocal sound became very aphonous, poor of harmonics, and it is a dangerous movement.
The correct movement must be like a crying sensation, very important for example for the develop of High Singing Formant, and it is not in relation with the closure of our soft palate.
The soft palate movements are indipendent from the larynx movements, and all these positions can be mixed.
About western style, we have listened to many overtone singers and if we pay attention we can understand what positions the singer are using.
Next time I will insert some images to explain better my ideas and we will look at the deep relation between overtone singing and opera siniging.