Phonetic Convergences

More phonetic convergences exist, but the diagram was formed as a response to Steve Sklar, who wrote about the sound of AUM. And I am quite convinced that the background for the reverence of this syllbale may be the organic sequence [ɑ]-[ɒ]-[ɔ]-[o]-[u]-[m] (as you proceed in the sequence the lips will approach and finally meet in m).
Likewise the convergence [a]-[æ]-[ɛ]-[e]-[i] towards [ŋ] is reflected in the mantra tradition as the AING mantra of Saraswati (as you proceed in the sequence the back of the tongue will rise towards the soft palate and finally they meet in [ŋ]).

It is a question how applicable the IPA chart is to overtone singing, because it illustrates the movement of two formants, while one of the keys to overtone singing is to adjust to one formant. But what I aim for here is to emphasize, that phonetics and overtone singing are not separate areas. And phoneticians refer to the IPA chart. Ultimately you cannot separate vowel characteristics, proportions of the vocal tract and the formant frequencies.

What I find important as a key to overtone singing - formants or not - is the awareness of the role of the primary places of articulation.
I consider the understanding of the nasals, specially [m],[n],[ɲ] & [ŋ], and the subtle opening from them into vowel sounds crucial to the theoretical understanding and the practical accentuation of overtones.

The picture is not complete, and I know that you may provide other techniques, but I find it very relevant for overtone singers to learn the basics of the phonetic language of which I am only a beginner myself.
And more generally it is a diagram for understanding the music of phonetics as it also deals with humming and rhythm.

The fact that you move two formants at the same time is also the explaination for the apparently contradictory convergences, as it would seem more obvious to let the back vowels converge towards the [ŋ]-sound and the front vowels towards the [m]-sound.

As you can see in the text beneath the diagram, I distinguish between which positions that are suited for overtone singing and on the other side convergences which are more for the sake of awareness in general.