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Slide guitar - acoustics question

Hello List,

Can anyone explain this slide guitar technique?

1) Lightly damp a single guitar string at, say, half its length to get the harmonic pitch an octave above the open string pitch (ie, interfering with the fundamental and odd modes of vibration),
2) Replace your damping finger with a steel slide at the same damping pressure, and slide it up the neck to some point along the neck.
The result is the expected ascending glissando from the beginning harmonic pitch to the pitch at the end of the glide, but also a fainter descending glissando that moves at the same speed and distance as the stronger ascending glissando. The downward glissando is easily audible with practice - more so if the rising harmonics are filtered out - and is also clearly visible on a spectrogram.

My students and their teacher have two questions: 1) if the slide really maintains the same pressure as the original damping finger, then theoretically its movement doesn't change the length of the string or the fundamental, but must instead be interfering with different vibration modes as it slides upward. So how does it produce a (chromatic) glissando?
2) what accounts for the descending glissando?

Thanks for any insights.

Nina Fales