hey guys
I have a great german book wich i can only recommend its called "Neue Jazz-Harmonielehre" (=New theory of Jazzharmony) written by Frank Sikora a british Jazztutor teaching in Switzerland he wrote:
my translation:
"to sing a tone and think of another while singing means to precive to 2 tones at the same time. You are listening polyphonic even though you sing only one. the other note is just a picture of you imagination.
When you say: "I am going to sing a maj7" then you hear the root note in relation to the one you are singing. you have to able alwys to hear more than there in reality is. the root note is vibriting within yourself (your imagination)"
In that chapter there are a few exercises. I f you like a post them too.
Its a espcially for composers a helpful tool. Just imagine a deaf beethoven who is writing the 7th symohony.
But Jazz- and Rockmusicans have it too. Once I asked Guthrie Govan if he's always while improvising hearing the line before playing it and he said that he is always imagine it the same way as hes playing it. As me for example, I'm often "surprising" myself with the lines i am playing. But I am working on it

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Just imagine the possibilities of creation with perfecting this ability. You could write whole songs without touching an instrument. Wouldn't that be cool?
In Ron Gorrows Book he is teaching this for the matter of transcription.