Here's how it works:

As long as new deposits keep coming in, they are used to pay out profits on old deposits.  The operator takes a cut of each payment too.

At some point new deposits aren't enough to fund the required payouts (because of inconvenient facts like there aren't infinite numbers of stupid people, or infinite numbers of Bitcoin in the world) at which point the operator makes a new forum account, and a new forum thread.

The early depositors double their money, encouraging them to play again
The operator takes a percentage of every deposit, encouraging him to run another Ponzi
The late depositors lose their money, encouraging them to get in early on the next one

Why wouldn't you continue doing this indefinitely?