The history of this sound goes back to the Kyma 5 days and discussions can be found on the old forum.  The core concept is akin to aural exciter that does not rely on amplitude to drive the harmonics / distortion. 

The PDU signal flow is Input > Filters > Wavefolder > Filters > Output Mixer.  

The filters are custom biquads filters.  Prior to the wave folder, the signal is transformed by the PilbertTransform.  The wave folder is a triple wave folder with variable topology.  The signal is then mixed back with a phase aligned version of the dry signal.

This sound can be subtle excitement of vocals and solo instruments to bring a "3d" quality to sounds or it can completely distort the sound.

Additionally, there is a second version that adds a noise gate driven by the original input. 

The wave folder uses is also published as a standalone sound called SkewFolder.

Original signal design by Pete.