With the iZotope Maximiser, you can set at which level (for all frequencies) it starts doing its stuff (Threshold), as well as the maximum level to which it works (Margin). The lower the Threshold, the greater the maximisation.
As for the internal mechanics of how it exactly does its stuff, other than being some sort of compressor-expander-brickwall limiter, I don’t know. There are several different algorithms, some of which are very CPU-intensive.