Of course this doesn't work.
What was funny was that as he started to explain his trick, he stopped, looked at me and announced, "I'm pretty sure I just lost you."
I looked at him and said, "no, I'm with you. I see what you are saying."
He replied, "yeah, but I lost myself".
We both laughed out loud. This is the kind of interaction you get with kids when you take the time to talk with them.
In the end, Larry was able to see how this trick works for adding but not for multiplying. Eventually he understood that when multiplying the 5 and the 9, if you reduce the 5 to a 4, you are really taking away 9 things. And if you increase the 9 to a 10, you are really adding 5 things. And that is why 4 x 10 = 40 while 5 x 9 = 45. A difference of 5.
I don't know whether I've explained this very well, but let me assure you that I got goose-bumps when Larry looked up with a fully illuminated light bulb hovering over his head.