Phasing seeks to reproduce the effect that arises when the speed of one of two tape recorders simultaneously playing back the same signal is altered. When the delay time of about 0.5 ms is slowly modulated, the changing phase relationship (i.e. the shift of one waveform relative to another) between the two signals causes a number of frequency bands alternately to be cut and boosted.
The result of phasing is a ’whooshing’ filter-like effect, not unlike that of a number of wah-wah pedals linked together, each affecting a different part of the spectrum. In a graphic display of the spectrum, the effect appears as a shifting comb, each ’prong’ corresponding to a resonance peak - hence the technical name ’comb filtering’ applied to the use of this range of delay times. (Source - Richard Dobson (1992). A Dictionary of Electronic and Computer Music Technology. Oxford University Press.)
See also:Filter, Flanging