Okay I give up, how the heck does this work? Okay, Daniel Berlinger sent me an explanation. I have to be sure it's correct, then I'll write it up. [Scripting News]
That's old, I knew it from my grad school days, though I admit the flash animation gives a great twist. If you have a two digit number nm then nm-(n+m)=n0-n which is constant on m. The trick is to put the same symbol in all numbers of the form n0-n. That is, 9 (10-1), 18 (20-2), 27 (30-3), 36 (40-4), and so on... Remember to write the numbers in a way that the diagonal is not clearly identified...
10:41:04 PM 
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