question about 'tonal' compressors

[Eddododo]

A bit of introduction-  there are, from a user standpoint, a few different types of tonal or stylistic  enhancement to be had from many circuits labelled all in ubiquity as compressors... in this case, i am a  bassist, and i am referring to non-utilitarian , non-surgical , and rather fattening or 'tonal improvement' type compression.

I understand the nature of these compressors in terms of  low ratio and smooth envelope qualities, but i want to know the mechanism ..                              it seems that the frequencies themselves are kind of 'normalled' so that, say, mids appear to be boosted       any good buzzwords, circuit examples or philosophy would be helpful...   Thanks!