There really isn't any "flavor text" in the book outside of the
clearly-identified in-character "quotes".
Spontaneous curses essentially consist of the user yelling "Hey!
Malevolent or wrathful spirits who enforce curses! Over here!
That's right, over here! Listen to me! I want you to go and get that
guy over there!".
That's why they're noted as invariably backlashing under various
circumstances and sometimes backlashing regardless of
circumstances. Since the person laying the curse is voluntarily
invoking it, they get no save - just like with any other bit of magic
someone uses voluntarily.
If a character is foolish enough to throw lots of spontaneous curses, they'll be hurting themselves rather than gaining an advantage. It's kind of hard to powergame by repeatedly bashing yourself in the head.