Poulenc gives a good clue to what it might mean in his writing for the piano, in which his observation of its complex sonority is matched by the tenderness with which he uses it. One can almost not bear to play the notes without the element of love that he clearly expresses yet without the slightest trace of sentimentality.
It is interesting to ponder this more closely: He often calls for dissonances within pianissimo chords, a seeming oxymoron in that the notation seems to promise a harsh reality whereas the dynamic indicates something more intimate. I conclude that he must be touching the edges of the strings' vibrations. Perhaps that is what composers always heard coming from the piano's strings.
To have written it down is an astonishing feat.
It may be one of the reasons why he is not taken seriously in certain circles.
Poor impoverished circles.