Provera works to bring on your period when you are having long cycles. So you take the Provera in order to O more often than you would if you just waited for AF to show up naturally.

For instance, I had a 90 plus day cycle recently. If I'd taken Provera to bring on AF at the end of each month, I'd have had THREE chances to get pregnant. But because I wasn't taking Provera, I only had one since you only O once per cycle. Provera simply mimics the hormones that your body should naturally produce in order to build up your lining and bring on AF.