A stereo tremolo: the level of the sound rises and falls in a steady rhythm. It is the oldest effect in the book and still one of the quickest ways to turn a held chord into a part.
The idea
A sine wave swings the volume of each channel. Spread is what makes this more than a volume pedal being rocked. With no spread, both sides dip together and the whole sound pulses. At 180 degrees the sides move in opposite directions: as the left dips the right swells, so the overall level stays fairly even and what moves is the stereo position. Settings in between give a pulse that also drifts across the image.
Rate changes the character as much as the speed. Slow settings are heard as rhythm. Past roughly ten cycles a second the dips blur into the tone itself as roughness, closer to a ring modulator's grit than to an amp's throb.



