Well, from my tests, I would say that it acts somehow like a threshold variable. 0 = off, and all other values = on, where lower values means more clamping (0,01 = extreme destruction), and higher values means less/softer clamping. For my test scene I found that values between 5 and 6 worked best.
Edit: Just did one more test, and actually I find that value 1 is working really well when "highlight compression" is off (1). The reason why 5-6 seemed better in earlier tests, was that HC was in action too.