That's a version of a classic financial-advisor scam. It operates on a similar mechanism: a set of predictions is sent to large number of marks. Roughly half

That's a version of a classic financial-advisor scam. It operates on a similar mechanism: a set of predictions is sent to large number of marks. Roughly half the predictions are in the money, the set of marks is reduced with each successive round to those who's previous "predictions" were accurate. At the end of the cycle, there are only a small number of marks left, but they're given the option to subscribe to future predictions for a handsome sum. Of course there are no further predictions.... There's also the apocryphal physician's trick of predicting a baby's gender (back when this wasn't trivially determinable in advance). The doctor would verbally give their prediction, and write it down in an envelope. Occasionally the parents would recall a different answer, but opening the envelope would confirm the gender of the infant. The trick of course was that was was written was the opposite of what was said. When what was said matched the delivery the envelope wasn't opened, so the secret was safe.