It intensifies proportionately based on the amount of metalic silver that is already present. With a "normal" negative you can get the equivalent of about N+1 expansion depending on the film. But it does essentially nothing in very thin areas where there is little silver to begin with. As is always the case, you can't increase shadow detail where none exists in the first place.
The Selenium should be used quite concentrated for negative intensification - usually 1+1 or 1+2. If there is enough density in the negative to begin with it works quite well for a moderate increase in contrast.
Agree with Mike. Selenium intensification can give you about one stop of contrast. It works to some extent, but don't expect miracles. It's to the degree of about half a grade of printing contrast, but with under-developed negatives (I assume properly exposed) every little bit helps.
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