The Distortion effects adds very harsh harmonics to the tone, simulating the effect of a guitar distortion pedal on the sound. A small amount of distortion will add a bit of 'bite' and compression to the sound. A large amount of distortion will change practically any tone into a nasty, overdriven sound.
The distortion effect has three controls:
Pre Gain sets the amount of volume that is sent into the distortion effect, from 1 to 200 percent. Sending greater than 100% of the signal into the distortion effect will cause even greater amounts of distortion to be generated.
Distortion sets the amount of actual distortion that is applied to the original sound, from 1 to 200 units.
Post Gain sets the volume of the signal after distortion has been applied. This control is useful for taming peaks that may occur because of distortion, to keep the post-distortion signal from clipping.