this was discussed earlier:
http://osdir.com/ml/audio.csound.user/2006-01/msg00278.html
regarding the excellent distortion technique covered by Hans Mikelson
in the Csound book instrument 3013.
However, he, does not mention any specifics about the technique used
in the Csound book article.

The waveshaping part of it is clear, but I wnat to understand better
the deltapi part of the instrument.
I've been looking into this technique again, and I would like to
pinpoint exactly what is happening.
It seems like the result could be described as a kind of frequency
dependent phase modulation.
It also seems like the duty and slope parameters exert a very similar
effect on the output waveform (but out of phase with each other,
naturally, as the corresponding control signals is out of phase). The
name "iduty" seems kind of misleading, as it is not related to duty
cycle modification any more than the slope parameter is. I'm not sure
if I can suggest a better name.
Both parameters generate sidebands at integer multiples of the
frequencies present in the input signal to the effect.

Maybe some of the dsp gurus here can try to explain what happens ?

best
Oeyvind