Go to this page:
http://www.euhein.blogger.com.br/leitordemente.html
The translation of the text is (btw, the page name means mindreader.html):
Think of a number with two digits (e.g. 54)
Subtract from the number the two digits (54 - 5 - 4 = 45)
In the table, check the symbol corresponding to the result (45)
Concentrate on that symbol and click the magic square....
Not very tough, but I found it interesting.