You can increase saturation of slide film (this is true of E-6 as well as it was of Kodachrome) with slight underexposure. By "slight" I mean 1/3 to 1/2 stop.If you go beyond 1 stop then it's just plain underexposed (unless the underexposed look is what you're going for).
Are you sure this works with scanners though? that might be true of printing it on ilfochrome paper but with scanners, it just means you get a dark image, the scanner can't see through the darker densities...
~Stone | "...of course, that's just my opinion. I could be wrong."~Dennis Miller