I didn't understand the rotation when I first tried, but I got some good advice from some members on here.
They told me that once you first practice them, think of it as the same rotation as a backflip, but just a bit off axis. So, in a backflip, you throw both shoulders back In a flatspin, try throwing one shoulder back while throwing the other forward. In essence, it is not a true flatspin (in my case because I was inverted), but it helped me out.
Before, I would throw it too horizontally, and it just didn't work. I eventually got them at the end of last year, but I still took a couple of hits first.