Operating as a Snap In for Kilohearts' Multipass and Snap Heap hosts as well as a VST/AU effect plugin, Ring Mod emulates an analogue ring modulator - that is, a circuit that multiplies the frequencies of two input signals and outputs the sum and difference of/between both.
In musical terms, it's a distortion effect that can be used for subtle brightening and enhancement or brutal sonic destruction.
Ring Mod's selection of secondary signals comprise a sine wave oscillator, low- and band-pass filtered noise, the input signal and a sidechain input, all shapable (gently or radically) using the Bias and Rectify controls. The Frequency and Spread dials set the oscillator or filter cutoff frequency, while Spread offsets the left and right channels for stereoising, and Mix enables parallel processing.
The oscillator and noise signals are useful and effective, but it's the Self and sidechain options that really bring Ring Mod to life, the latter giving total freedom of source signal selection.