Disney sends out from time to time, either postcards or email postcards, to select folks, offering some very good rates for specific times of year. To use these codes, you must be the person listed on the card and have the pin number in hand. We have not been able to figure out exactly how Disney decides who gets these cards. You just have to get lucky I guess.
As far as AP rates go...these are based on having an annual pass, in hand, when you check in. You can get the rate when you make your reservation without an AP but technically you are supposed to be able to show the CM your AP when you check-in.