The intro loops indefinitely until you attack an enemy, at which point a drum fill triggers and the main song kicks in. If your health gets to 2, everything but the bass and drums are lowpassed, which is even more intensified at 1 hp. Also, there's a hidden "coolness" meter, which increases when you dodge attacks, land hits, and kill, and decreases when you get hit. When that hits a certain threshold, the background music hits a new section. (I may have made the threshold a little too high.)
It's a little subtle, and intentionally blends into the background well, but the music is adaptive. Perhaps I should've outlined that in the description...