I made it all the way through Dark Souls 1 and 2 and about half of 3 before I even knew that was a thing. I was getting curbstomped by Dancer of the Boreal Valley and went online looking for discussions about her. Lo and behold:
this video is a re-upload because it looks like the original was removed
Tl;dw - Dancer's song is in 3/4 time instead of 4/4 and she dances with her music. This gives her a crazy pattern that people always get got by because what feels like an opening actually isn't. In order to defeat her you have to listen to her song and learn to dance with her.
Once I learned that it opened up an entire new world of understanding across every soulslike game I played and immediately halved my average number of boss attempts. No joke. Not every boss can be beaten blindfolded by just listening to their OST but it'll give you good timing cues for the fight more often than it doesn't.