What are the key concepts and principles of retrogression in music theory?

Retrogression in Music Theory refers to the movement of chords or harmonies in a piece of music in a backward or descending direction. It involves the use of chords that move in a way that creates a sense of tension and resolution. The key concepts and principles of retrogression include the use of chromaticism, non-diatonic chords, and dissonant harmonies to create a sense of instability and complexity in the music.