Provera or medroxyprogesterone acetate is a hormone frequently given to women in order to achieve regular cyclic bleeding. Physiologically, this works well because it mimics what happens naturally during a normal ovulatory cycle. If you break the menstrual cycle into two parts, the follicular phase where the egg is developed and luteal phase, where the egg is fertilized and implanted, you may understand why cyclic provera works. In a normal cycle, during the follicular phase, a group of eggs develop in the ovary. That creates estrogen. That estrogen in turn stimulates the lining of the uterus to proliferate, or develop. At midcycle, the developed egg is released or ovulated.
Now comes the luteal phase during which, the mechanism that made the estrogen during the follicular phase, starts to make progesterone which maintains the uterine lining throughout the luteal phase. That little progesterone factory lasts 14 days and unless rescued by a conception (pregnancy), involutes (dies). The subsequent sudden loss of progesterone results in a "withdrawal bleed" or menstrual cycle. A 10 day course of provera or progesterone mimics that little luteal phase progesterone factory. The subsequent withdrawal of the oral progesterone mimics the natural withdrawal of progesterone and should trick the uterus into menstruating. The patient that doesn't ovulate regularly such as the patient with polycystic ovaries, never develops the little progesterone factory, thus the lining never has a progesterone withdrawal. In the absence of the regular orderly withdrawal, the uterine lining becomes unstable and unpredictable, susceptible to sporadic breakthrough bleeding.