It is a multi-band nonlinear compressor. It deals with each band of hearing (https://en.wikipedia.org/wiki/Bark_scale) individually. In each band, it elevates soft sounds to just past the threshold of audibility with someone's impaired hearing, to make it sound like the softest sound they can hear. And it elevates medium sounds a little bit less, to make it sound like a medium sound. And it elevates loud sounds not much at all. Hence the nonlinearity.