Playing with feedback in the Vixid VJX16-4. Having an absolute blast! This effect works without any input at all, but a camera feed gives us something recognizable and understandable.

Routing:
All outputs on Master1, all connected back to inputs in feedback loops.
Track 4: Camera input. Contrast tweaked to Max. Layer on the bottom (D), opacity 100%. Face visibility tweaked with Gain control.
Track 3: Master feedback input. Blend mode: Difference. Layer Opacity ~40%, second from bottom. Gain max.
Track 2: Master feedback input. Blend mode: Normal. Layer opacity 25%. Gain 75%. FX: Flip horizontal and Negative.

The horizontal flip, negative layer, and negative blend mode are all working together to create this effect. I think those are the magic settings to get this kind of effect working, but I'm too scared of losing it to change anything around at the moment.

More information: vixid.noisepages.com/2008/06/feedback-101-introduction-and-demo-of-feedback-on-the-vjx16-4/