Add a fixed number of hertz to every partial in a file. Unlike a pitch shift, which multiplies, a frequency shift moves everything by the same absolute amount, so whatever harmonic relationship the source had does not survive.
Shift
The shift runs from -1000 Hz to +1000 Hz in tenths. Positive moves the spectrum up, negative moves it down.
The size of the shift decides what kind of effect it is. Under about 5 Hz the sound stays itself but stops sitting still, because no two partials keep the same interval. Between 20 Hz and 200 Hz it turns tonal material into something bell-like or metallic. Past that the source is unrecognisable and the shift is the sound.
Drums and speech take small shifts better than sustained tonal material does, because neither depends much on a harmonic series in the first place.
Single sideband
Multiplying a signal by a carrier produces two copies of the spectrum, one moved up and one moved down. That is a ring modulator, and the mirrored copy is a large part of what makes one sound the way it does.
A frequency shifter cancels the mirror. The signal is split into two paths held 90 degrees apart across the audio band by a pair of all-pass networks, then recombined against a quadrature oscillator so one sideband adds and the other subtracts to nothing. Only the wanted copy remains.
Feedback
Feedback, up to 85%, returns the shifted output to the input so each pass is shifted again.
One pass moves a partial by the shift amount. With feedback, that partial also appears at twice the shift, three times, and so on, spreading into a ladder that grows out of the original. Small shifts with high feedback are where the endlessly rising sweep comes from, because there is no octave for the ladder to repeat at.
Stereo offset
The stereo offset, up to 50 Hz, adds to the shift on the right channel only.
The two sides then move at slightly different rates and beat against each other continuously. Even a fraction of a hertz is enough to stop the image sitting still, and 5 Hz to 20 Hz opens it up noticeably.
On a mono file the offset does nothing, since there is no second channel to run at a different rate.
Example: a sustained pad that needs to stop repeating
Shift 2.5 Hz, feedback 45%, stereo offset 0.7 Hz, mix 60%.
Nothing moves far enough to change the note, but no partial holds still against another, so the pad drifts continuously and the two channels never quite agree about where it is.