While singing coaches offer elaborate physiological background for what's going on behind the exercises, when it comes to voice coaches for actors or techniques for opening the voice I can't find any info on this.
I talk about exercises like vocalizing an Ahh.. sound when you try to release the voice, sometimes while moving the pelvic or as in this example:
Can this be explained or understood using common voice theories like SLS etc.?
The physiology behind speaking and singing is the same, which is why you haven't been able to find separate answers for acting. It's all vocal physiology. All of it uses the human voice. Singing could also be described as sustained-vowel speaking, which is what is happening in the videos you posted.
One simple tip that helped me a lot was imagining a big rotating circle, the note leaves my mouth as a stream of energy and re-enters my body at the abdomen. It's not a scientific fact, it's a visualization. It helped me much more than being repeatedly told to "sing/speak from the diaphragm".
I agree that visualizations are the most important for a singer/actor. The voice must be kept in the "front" (or mask) by hearing it as an "ugly" voice through your inner ear. Another good way is to think about "singing from the tongue muscle" to get more air into your singing or speaking and attach the voice to your body.
No human can live without a diaphragm, because no human can breathe without a diaphragm. No human can sing without a diaphragm, because it is the diaphragm that causes the lungs to fill with air. The diaphragm is critical to singing.
Any exercise that develops control over the diaphragm is likely to improve the singing (or speaking) voice. We can't will our voice to get louder. We can only pump more air. It's worth keeping this in mind when assessing the advice of voice coaches.
