From the Serge Catalog:

The Serge Modular WAVE MULTIPLIERS (VCM) provide a new link... representing an advance in synthesizer technology. In this typical patch, the Wave Multiplier could be placed just before the VCF. Like the VCF. the Wave Multiplier affects the timbre. Unlike the VCF, whose action is a subtractive process of filtering frequencies from the input waveform, the Wave Multipliers are able to dynamically process the input waveform to produce new harmonically-related overtones... The input sound comes out richer in harmonics, somewhat similar to pulse-width modulation and to linear frequency modulation, but with a new characteristic timbre. The nearest we can come to describing the unique sound qualities (there are three different sections) is to say that they alter the timbre in exciting new ways, producing interesting alternative forms of signal processing which are unique in the Serge Modular Music System. Since there are three entirely separate and different types of Wave Multipliers in this module, an enormously varied palette of new effects can be synthesized.

The uppermost section has two switchable effects. With the switch set at the "HI" position, the module functions to "square-up" an incoming signal With the switch in the "LO" position, the module is a linear gain controlled VCA.

The middle Wave Multiplier provides a sweep of the odd harmonics (1, 3, 5, 7, 9, 11 and 13th)

The bottom Wave Multiplier performs non-linear wavehaping known as full-wave rectification, but with sophisticated level-compensating conditioning as well.