The ES04 is the simplest of the three Wave Multipliers, but it has two switchable effects.
With the switch in the 'OVERDRIVE' position the module functions to 'square up' an incoming signal. This is not the same as a simple comparator squaring function though since there is a rounded flattening of the signal peaks, an effect somewhat similar to overdriving a tube amplifier, except that this process is voltage controlled.
With the switch in the 'LINEAR VCA' position the module functions as a linear gain VCA. This is useful for various functions such as amplitude modulation and for gating signals on and off.