Beat phenomenon question

[Richard Moody]

While experimenting with a digital "pocket" tuner (Korg DT-1) and Tunelab, I fed the
Korg's audio output into Tunelab to be "measured".  I discovered that when Tunelab
was in the audio mode both tones could be heard. That got me to wondering what a
beat of one cent sounded like.  It was faster than I imagined, so I tried .1 cent. 
Veeery sloooooow.   While I was marveling that a beat of  .1 cent could be heard I
noticed "pulsation" of sound seemed to be on one note while the volume of the other
note sounded steady. 
	The Tunelab puts out what I think is a sine, while the Korg gives a sawtooth. In
the beat pulsation the sine seems to change volume while the sawtooth stays the
same.  When the volume of the sine sounds low I can release the sawtooth and the
volume returns to the sine.  When I put back on the sawtooth the volume of the sine
returns to where it would be if I had not turned off the sawtooth.  At some points
in the beat cycle the volume of the sine doesn't change when I release the saw, and
it never seems to get less when I release the saw. Thus in the beat cycle it appears
the pulsation is caused by the changing of volume of only one of the notes.  Is
there an explaination for this, and why one note and not the other? 
	Also why does it seem when one note is released while the other is held down, that
the beat cycle does not start over when the released note is played again. It seems
to continue in its place even if I let it up for 3 or 4 seconds before playing it
again.  

I don't know what this has to do with piano tuning, but that is what I got for
wanting to hear one beat in 20 seconds.  ---ric