You can setup the relationship between the input and output level of each sample in the Overdrive Spline Window that can be opened from the vapor panel. The vapor parameters are controlling the amount of modulation of that spline curve.  The modulations ranges from 0% (“linear” - meaning input=output) to 100% (the current spline).  

The blue line in the Overdrive Spline Window depicts the current applied modulation of the first generator (1).

Note: By using the “Phase between Generator” Sliders you can assign different modulations to different generators.

The amount of Overdrive Modulation is displayed in the Cloud 3D window by a saw like top of each grain.