Auto Panner moves a sound back and forth between the left and right speakers. A sine wave sets the position, so a single mono source comes out as a stereo signal in motion.
The idea
Panning is the oldest way to make one sound occupy two speakers. A panner builds the two channels itself, so this works on a mono guitar, voice or synth without needing a stereo source.
Rate behaves like two effects. Below about 8 Hz your ear follows the movement and hears a position travelling. Above about 10 Hz the movement is too fast to track and turns into a rough texture with a stereo edge. The range goes up to 20 Hz so that end is reachable.
