It's pretty simple really.

Let's say, for example, that on Day 4 we post the name Randy Petersen at the top of this page. Randy then has 24 hours to send a private message (PM) to the 12 Days member account claiming his prize. Well, 24 hours comes and goes and there's no PM from Randy claiming his prize (sure, it seems odd, as Randy is ALWAYS up on FT, but we’re sure he has his reasons).

Then, on Day 5, the username Janet at WebFlyer is posted. Janet sends in a PM and collects her 5,000-mile Day 5 prize AND the unclaimed 4,000-mile Day 4 prize, for a total of 9,000 miles (and shortly thereafter, it is discovered that Randy was briefly kidnapped by a group that can in no way be traced to Janet. Thankfully, he is released unharmed ).

This playing forward of unclaimed prizes could occur over the course of several days. For example, if the Day 2 and Day 3 prizes had also gone unclaimed in the scenario above, Janet would walk away with a total of 14,000 miles -- and that's not including the Day 3, 5,000-mile bonus prize she would also earn if she were a FlyerTalk Premium member (as it would have been unclaimed on Days 3 and 4).

Quick and extremely relevant fact: There are over 85,000 registered FlyerTalkers, many of whom are not active on a daily basis.