I didn't look all the way through the USNW site just now: there's a lot there! However, from what I did see I don't think they said much about the vibrations of the body below the 'main top' resonant mode pitch. There's one vibration there that involves the neck, and can effect the tone.
You can think of the guitar as a sort of odd-shaped xylophone bar. As with any such thing, the lowest vibration mode is one in which the two ends are always moving in the opposite sense from the part in the middle. In the case of the guitar, the stationary places that demark the 'ends' from the 'middle' (the 'nodes' of vibration) for this lowest vibration mode are roughly at the nut or first fret, and across the wide part of the lower bout. You can hear this vibration if you hold the guitar up by pinching the neck at the first fret or so, deadening the strings, and tapping on the back of the headstock while you allow the instrument to hang freely. You may need to put your ear right up to the surface of the headstock. This is often a low pitched note, and can be around C below the low E. The pitch of this vibration depends a lot of the stiffness and mass of the neck, and the mass of the headstock, and we often call it the 'neck mode', even though the whole guitar is vibrating.
Generally speaking, the 'main air' resonance wil be the lowest pitch mode that can radiate sound effectively. This is often around G on the low E string on guitars, and seldom much below F. The 'neck mode' is, as I say, often lower, and does not by itself usually produce much sound. However, the neck mode can cause the top to belly in and out, and move air through the soundhole. If it is high enough in pitch it will 'couple' with the 'main air' resonant mode. In that case what we see in the sound spectrum is a double peak around the pitch of the air resonance, which will typically be lower in amplitude and broader in pitch than the 'air' peak by itself would be. In a sense, the area under this spectral curve is the 'total available horsepower' in that range, and it's greater when the neck mode is high enough in pitch to couple. The guitar takes on a particularly 'rich' or 'dark' bass tonality, with the notes being even and powerful.
It's hard to 'tune' this mode on a completed guitar. Using a light wood, such as cedro, for the neck seems to help in moving the pitch up. A 'tapered' neck that is deeper at the body end will usually have a higher pitch. The weight of the tuning machines can make a difference, which is one reason swapping out machines can alter the timbre of a guitar. A thick fingerboard might help raise the 'neck' pitch. Carbon or other stiffening reinforcement of the neck can help if it does not add a lot of weight. On one short scale instrument I made, the neck mode was higher in pitch than the air, and I had to add lead weights inside the headstock to 'tune' the neck down. This is unusual: normally ther neck mode is too low, and there's not a lot you can do about it.