Based on the nonlinear compressor but does the opposite. Uses the envelope follower to apply a nonlinear expansion. Rather than controlling the volume directly the envelope modifies the nonlinear transfer function to apply more or less loudness/saturation. When the envelope is low the transfer function is linear (low gain/saturation applied to quiet signals), when the evelope increases the curve is pushed towards a a heavy tanh saturation. So quiet signals are unsaturated and loud signals get saturated.