It has been my impression for a number of years that sensations in the mask are connected to phonation and not resonance adjustment. However insofar as intensity of resonance sensations depend on efficiency of phonation, good resonance habits (e.g. appropriate vowel adjustments for register rotation) are reinforced when suddenly phonation is coordinated appropriately. This is the source of the confusion.
Because the fold edges themselves are not innervated, it is not possible to sense them. The breath pressure that results from a well phonated tone is felt both as a feeling of support in the body but if the external inter-costal muscles do not do their part to reduce the subglottic pressure, that pressure is often sensed in the form of the contraction of unnecessary extrinsic musculature of the larynx, often perceived as a vertical straining muscle in the neck (Sterno-cleido-mastoid).
Another set of sensations are induced by bone conduction and are in fact felt throughout the body. Some singers sense vibratory sensations in the chest, back, head, etc. The specific sensation in the mask is somewhat complex. It is my belief that the nature of the subglottic air pressure is transferred in the mask when the phonation is relatively efficient. If there is enough intensity at the glottal level that does not require noticeable tension in the extrinsic laryngeal musculature, then the glottal intensity is experienced as a mask sensation.
I have asked the scientists about this sensation, but I have not gotten a sense that they have figured that one out yet. I am certain that the sensations we sense in the mask are phonation-based. In terms of the singer’s experience, the important thing is that this does not get confused with resonance in the sinuses, etc. The sinus cavity is not a viable resonator as has been discussed at length by William Vennard (Singing: the Mechanism and the Technique) and not refuted.
When mask sensations are attributed to resonance adjustments, singers often try to change sensations in that region, thereby altering phonation coordination unfortunately. The efficiency of phonation is the one aspect of singing that should remain unchanged. The sensation of the sound emanating from a point in the mask is a good one that is suggested by many teachers. This is the sensation of efficiency (depending on how that sensation is achieved). This maintenance of this efficiency depends on an ever-changing resonance mechanism. The consistency of the tone depends paradoxically on an unchanging efficient processing of air through the glottis which depends on a continuously changing resonance mechanism.
This would also explain the fact that many teachers who work with mask sensations couple this element with the necessary release relative to space in the throat and soft palate. The vocal tract partitioned by the tongue constitutes the two primary vowel formant spaces. It makes sense therefore that mask sensations (phonation) coupled with adjustments in the vocal tract would yield super results.
The only caveat to which I alluded above is the manner by which the mask sensation is achieved. Often to achieve the glottal efficiency that produces the mask sensation, the singer produces a supra-glottal squeeze explained in this video already linked to a previous post on phonation. It is important to keep in mind that the supra-glottal squeeze is a compensatory mechanism for glottal inefficiency. It is also interesting that the necessity for the squeeze is removed once pitch is raised. This explains the mechanism that occurs when one attempts to speak or sing lower than is naturally efficient. On a personal note, I may have been guilty of this when I sang as a baritone. If my natural color is that of a tenor, my innate sense of what a baritone should sound like may have induced an unconscious squeezing to achieve that sound. By raising the pitch, the squeeze goes gradually away and the true, lighter and more efficient nature of the voice returns.