RezzoEQ has two what we call "Spring Resonance Filters". This system uses a very fast spring resonance algorithm that is controlled by the input audio signal and frequencies, that acts as either a low pass or an high pass filter.
Some of the properties of these "springs" can be adjusted with the three smaller knobs on each filter. The 'Energy' and 'E-Curve' knobs set how much energy a spring accumulates as well as how fast it responds to changes in the input audio frequencies. The 'Damping' knob sets how fast the self-oscillations will die out. As an example: setting the damping to very low, with high energy settings, will make the filter "sing" as it constantly self-oscillates while roughly following the frequencies of the input audio.
To control the resonant frequencies RezzoEQ has build in tanh distortion which automatically limits the output volume in a smooth but dirty fashion.