The supposedly secret dark art of mastering is actually nothing of the sort; it’s just good ol’ practice and experience. A mastering engineer learns how listen to the material in a certain way, to hear its spectral and dynamic makeup and have an understanding of what, therefore, should or should not be done.
They will have intimate knowledge of the way in which different processors respond to and colour audio, and will have developed specific approaches for different types of music, as well as knowledge of the technical requirements of different publishing media and platforms. So there really is no mystery to it, just an awful lot of expertise that takes time to develop.
