This control lowers the level at the input of the eq. Ideally, this control should be set at 0dB to maximize the signal to noise ratio of the unit. However, if you are adding lots of gain with one of the gain controls you may be clipping the eq internally - you'll hear distortion when this happens. In this case switch the input pad to -6dB or -12dB. By lowering the level at the input you'll prevent the distortion.