The Delay knob sets the average delay time. The Speed and Depth knobs control the modulation.
The higher the Feedback position, the more effective the effect is.
If the Stereo button is engaged the delay times of both stereo channels will be modulated with out-of-phase signals. With low delay settings this sounds like a rotating speaker. Reverse St. button is identical, except that the signal will 'spin' in opposite direction.