Auto swell generally, a rise in level from some starting level to a final level when keyed manually or electronically. Although the musical implications are more involved than this simplistic explanation, for a simple sine wave, symmetrical clipping generates only odd-order harmonics, giving a reedy, or raspy sound to the resultant waveform. The hardness or softness of the clipping matters. Some solid state devices actually flat top, then invert, producing a hollow topped output waveform at hard clipping. There is a continuum of clipping hardness, depending on the circuitry used to clip.
Soft clipping emphasizes the lower- order harmonics, the third and fifth, etc.
Hard clipping has a mix slewed to the higher order seventh and up harmonics, which are harsher sounding. Intermodulation distortion, the production of sum and difference frequencies from frequencies in the input waveform, varies with the amount and hardness of clipping. Intermodulation sounds harsh and ugly. The amount of intermodulation is a characteristic of the circuit that produces the distortion. This causes the generation of both even and odd harmonics, in contrast to symmetrical clipping's odd-order only.