Whoa! For real this time

Wow. Sorry if anyone tried to click the online mind reader link from yesterday's post. I apparently put in a broken link. (Thanks to my mom for letting me know). Anyway, I am reposting the entire thing below, with a real link this time. Sorry! This is off topic, but this online mind-reader was really freaking me out. Check it out, then come back and click through below to get the explanation. I felt dumb when I realized what the trick was.

From the first comment here:

This ‘magic’ is based on multiples of 9. The answer to the 2-digit formula is always a multiple of 9. On the board, the signs change order, but the sign on the multiples of 9 (9, 18, 27, 36, etc) are always identical.