Well, it was supposed to be sincronized with the music and if you deal enough damage before the music ends, it will play the next part instantly and no cut will be there so for playing the best way possible you get the reward of the full song without any cuts
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Yeah, but... that doesn't actually work? You don't actually face any in game reward for beating the boss faster, it only gives you a somewhat normal musical choice. I know what you're going for, but it simply isn't going to work. It punishes the player for not beating the boss in an exact amount of time, not in game, but by making the experience directly less enjoyable.