I wonder if the modulation is a rather extreme form of tremolo, which is a regular variation in level. Now, giving it a name doesn't explain how she does it, but in two-reed instruments it's a product of tuning them slightly differently, and as a result producing beats. The question then becomes how this could be produced with vocal fold vibration, in particular, how the vocal folds could be caused to vibrate at two frequencies at once about 25 Hz apart. I haven't got a clue, but suspect that it is related to the low F0 she produces at the same time.
The oscillation of the vocal folds is a physically complex phenomenon, and both in numerical simulations and in real observations, two or more different periodicities can sometimes be observed (though I have no idea whether that's what's behind the observed low-frequency modulation in Ms. Abramson's voice).
From a clinical perspective, my understanding is that this is what diplophonia is. See e.g. Shigeru Kiritani, Hajime Hirose and Hiroshi Imagawa, "Vocal Fold Vibration and the Speech Waveform in Diplophonia", Ann. Bull. RILP, 1991, who discuss different frequencies of vibration in the left and right vocal folds. I believe that similar differences can sometimes occur between the front and back or top and bottom portions, though obviously these subparts are less well differentiated than the left and right portions are.
From a certain point of view, the formation of beats, at a frequency corresponding to the difference of the frequencies of two summed sinusoids, is mathematically trivial, as this fragment of Matlab (or Octave) code illustrates. We create two sine waves, one at 120 Hz and the other 165 Hz, and a third that's simply the sum of the two:
x = 2*pi*(0:11024)/11025; y1 = sin(120*x); y2 = sin(165*x); y3 = y1+y2; subplot(3,1,1); plot(y1); subplot(3,1,2); plot(y2); subplot(3,1,3); plot(y3); wavwrite(y1',11025,"Beat45a.wav"); wavwrite(y2',11025,"Beat45b.wav"); wavwrite(y3',11025,"Beat45c.wav");
This produces the following plot of the three waveforms, in which the summed signal shows the apparent modulation of amplitude at a frequency corresponding to the difference of the two sine waves that make it up:
And the corresponding audio clips (recoded as mp3 with onset and offset fades to avoid clicks):
|y1 (120 Hz):||
|y2 (165 Hz):||
We can also take the trigonometry in the other direction, so that (as Wikipedia explains in the article on Amplitude Modulation), when we modulate the amplitude of a carrier c(t) with a lower-frequency signal m(t),
y(t) can be trigonometrically manipulated into the following equivalent form:
Therefore, the modulated signal has three components, a carrier wave and two sinusoidal waves (known as sidebands) whose frequencies are slightly above and below ωc.
The corresponding vocal-tract physics is somewhat less trivial; and that lack of triviality extends to explaining how (what is probably) the same phenomenon can sometimes result not in a regular low-frequency modulation but in "vocal fry" — a state of chaotic vibration without stable periodicity — as was illustrated in my earlier post in the last syllable of Ms. Abramson's pronunciation of the word "schoolkid":