It's more or less just a progesterone pill. You take it for usually 5 (sometimes 10) days, and then you get a withdrawal bleed if you have absent periods. I took it when I had an extremely long bleed, and it stopped the LOOOOOOOOONG cycle I was in, and then started a new one a few days later. The only reason that periods are heavy with it sometimes is because the progesterone signals your body to build up the uterine lining... on top of what is already there... and when you have the withdrawal bleed, there's extra "stuff" to get out of you. I had to take it after my chemical, and it was fairly heavy, but not painful. I think it depends on the person!
