The sphenoid sinus is at the top of the pharynx.

Stick your finger in your mouth and touch the soft palate (try not to gag) and right above it is where the sphenoid sinus is located. It's in the middle of  the head and is the "spot" that is highlighted in Vocal Wisdom: The Maxims of Giovanni Battista Lamperti. It's the place where vowel and tone must start.

Why does vibration always "hit' the same spot at the top of the pharynx? Because there is an open path to that spot. What helps me to "feel" the start of vibration at this post-nasal spot in the head? The sympathetic reverberation of the middle sinus in the skull—an enclosed cavity in the head directly above the pharynx. In fact the bony structure of the skull reports all that happens in the throat.

—William Earl Brown, Vocal Wisdom: The Maxims of Giovanni Battista Lamperti (1931): 113.

Is it real, this resonance? Nope. Sorry. If we drilled a hole in your head and put a probe into the sphenoid sinus, we would not find that it resonated when you sang with all the ring you could muster. Voice scientists are right on that score: the vocal tract really is the only resonator.

However, the spine does conduct sound/vibration into the head from the larynx and vocal tract. It's called bone conduction (the scientific term for this phenomena is "forced resonance"). It is felt, and one of two avenues whereby vocalization is perceived by the singer. It is anticipated by the singer who has a highly developed feel for his or her voice. This is why many teachers say you should "feel" yourself rather than "listen" to yourself. What they may not know, however, is that the audition of bone conduction is a vestibular function of the ear, and is self-listening of the highest order and importance to the singer, which the Old School called voice placement.

What vocal technique did the Old School use to develop awareness of the "spot"?

Long practice on mezza voce, which Clifton Cooke—an exponent of the García School—called the centrale voice.

Find the center of your head and you have found your voice.