http://www.iua.upf.es/~taussenac/wahwactor.htm
The Wahwactor is a two-input and one-output system. Out of the two input tracks, one of the tracks may be considered a control rather than a proper input since it is the one in charge of driving the transformations to be applied to the audio signal. In the context of the Wahwactor, the audio signal is typically a guitar signal and the control signal is a voice [wa-wa] utterance signal.
First, the voice signal is analyzed to pick up a meaningful descriptor that, after a simple conversion (shift, scale and smooth), is used as the centre frequency of the wah-wah filter, through which the guitar signal is sent to be mixed with the original.