Vortex Sound Research

I'm trying to understand what exactly the "drive" function does on the ASM Hydrasynth. Intuitively, from just playing around with it, it sounds like it raises the amplitude and has a sort of distortion-like effect as it gets turned up high. But then on other patches, it seems like it has little effect. The manual doesn't say much about what it's doing other than "adding warmth or grit to a patch". Can someone describe in more detail what it's doing to the waveform to create this "warmth or grit"?