How your payment is calculated:

We use a Binarized Scoring Rule. Your prediction is used to calculate an "error score." The closer your prediction is to the true likelihood, the lower the error score. The computer then draws a random number between 0 and 1. If your error score is less than that random number, you win the prize. Making your error as small as possible maximises your chance of winning.

Mathematically: This mechanism is "incentive compatible." This means that your expected earnings are highest when you report your true belief. There is no way to "game" the system; being honest is your best strategy.