Whoever claims that the retracted tongue is not important is claiming false science and fact. The tongue retraction helps keep the larynx low and also, when done correctly, the hyoid bone stable. That is crucial. It also opens up the pharyngeal space.
Ignorance about the correct use of the tongue abounds. That is so unfortunate because getting the tongue functioning correctly is *CRUCIAL* to getting the most efficient and beautiful chiaroscuro sound. Some of the most respected voice teachers are telling students that the retracted tongue tip is wrong. They are limiting if not damaging voices.
The retracted tongue is natural. You will see it in young children when they sing. And sing well. Manipulating it forward or behind the teeth lifts the larynx up and forward which goes against the proper functioning of the hyoglossus muscle. The hyoglossus muscle retracts the tongue making it more convex. The hyoglossus connects the tongue to hyoid bone. The the larynx is connected to the hyoid bone.
Also the geniohyoid will pull the hyoid bone up and forward. This muscle connects the chin to the hyoid bone. So it lifts the hyoid bone which lifts the larynx up and forward. You will notice that many singers that hold the tip of the tongue behind the bottom teeth have a protruding chin. This is because of the wrong functioning of the hyoglossus effecting the geniohyoid.
This also makes dropping the jaw cause tension in the mandibular area which is one of the causes of TMJ. If someone doesn't get TMJ fully it will make that area sore and tense.
Here are some examples of singers retracting the tongue. Some of these singers I think are talented, but need work on other things. However, this is about the tongue in particular:
Watch from 2:57
Hadley, who was taught by my teacher, watch at 2:26
In Jerome Hines book 'Great Singers on Great Singing' Birgit Nilsson, the Swedish Wagnerian who said outright she does not hold the tip of the tongue behind the teeth. Watch at 2:30. You see the tip of her tongue retract away from the bottom teeth:
The same thing with Melchior. Watch at 55 seconds:
Juan Diego Florez is not my personal favorite because of other vocal issues, but he does this as well.
Watch from 1:20:
Also not my favorite tenor, but he retracts also:
And one of the top rated singers today Jonas Kaufmann, who has a ton of vocal issues, but at least does this correctly. Watch from 3:12