“A phaser is an electronic sound processor used to filter a signal by creating a series of peaks and troughs in the frequency spectrum. The position of the peaks and troughs is typically modulated so that they vary over time, creating a sweeping effect. For this purpose, phasers usually include a low-frequency oscillator.” Wikipedia

*Unofficial documentation

source: http://en.wikipedia.org/wiki/Phaser_effect

Credit: Guitarix team