The guards would then use the virtual cash for their own means, including trading it for real-world money. Dali claims he overheard guards bragging that they could make close to $1,000 a day off of the efforts of the inmates, none of which ever made its way into the hands of the workers. He also claims that certain quotas were set, and that those who didn't raise enough virtual cash would be physically beaten.
Raising large amounts of credit in online games through the use of multiple accounts and individuals is known as "gold farming." The practice is typically frowned upon due to the nature of the work and the fact that those involved are usually paid very little. The problem is especially widespread in China, where the government was forced to ban the practice in 2009, though it continues to be an issue to this day. Chinese officials have denied the allegations, and insist that because playing an online game would constitute "contact with the outside world," prisoners would never be allowed to engage in such activity.
Guardian via NPR