Bitwig's own tutorial channel walks through oscillator feedback in Phase-4, showing how routing a sine wave's output back into its own phase input progressively distorts the waveform. Starting from two identical oscillators at a 1:1 ratio, the demonstration shows how increasing phase modulation from one oscillator into another shifts the spectrum toward a sawtooth-like shape, with harmonics present but in different proportions than a standard sawtooth.
Self-feedback is a step further: because the modified output becomes the new input, each cycle feeds on the previous result rather than a clean sine wave. That compounding effect pushes things toward instability faster, which is where the sound design potential lives.
The second half of the bit addresses how to keep that instability musical. Mapping velocity to the feedback amount means softer notes stay clean while harder strikes push into harmonic complexity. Layering a slow LFO on top, set to a two-bar cycle, keeps the parameter moving gently over time so the sound breathes rather than sitting static.
The underlying idea applies broadly in Bitwig: animating feedback parameters slowly, rather than setting them and leaving them, tends to produce more interesting results. Velocity and LFO modulation together give you both expressive per-note control and ongoing movement.