"I was the one who wrote this in a previous comment regarding the rationality of the fear of darkness. I think this definition(Which I learned from Eliezer) is in fact useful: if you know of two procedures A and B where A is correct according to some standard of rationality but will make you lose whereas B will make you win I would choose B."

That's the point: it's knowing that B leads to winning, and acknowledging that winning is the goal, that makes choosing B rational.

"Knowing that the tarot decks will "make you win" is reason enough to use them, no matter how irrational that may appear."

If we establish that we want to predict something, and we acknowledge that tarot is correlated to whatever we want to predict, using tarot to predict that thing IS COMPLETELY RATIONAL. We do not need to know the mechanism behind the correlation. What we DO need is to be able to look at our evaluation of the tarot's usefulness and determine that every step in the reasoning is correct.

The reasoning that concludes looking at tarot is an effective way of predicting [whatever] is fairly simple and trivially easy to verify.