Rant: Where did the 10 second rule for computers come from?

One of the biggest mysteries that I've run into ever since I've started computers with the legendary rule:

“Power the computer down, wait 10 seconds and restart.”

Or something to that effect.

I remembered one of my first computers that was ever declared mine (since I was too young to still have a credit card), and setting it up in my room. My parents had since strayed away from computers and the dot-com boom was a recent tragedy to the internet. I can still remember the fresh instruction papers from Gateway that looked like a Sunday morning cartoon newspaper that told me to assemble my computer in the way the told me. I felt bad for the person who couldn't do it as the most of the instructions were actually pictures instead of words. But as I got my computer up and running I remember seeing the troubleshooting guide telling me that if my computer locked up to “depress the power button, wait for 5 seconds until off” and then waiting like a kid counting down the days until Christmas, wait yet another 10 seconds until pressing the power button.

Being as rambunctious as I was, I decided to try it out. First, I waited the 10 seconds and tried it. After that, I would then try pressing it right after it powered down.

Nothing changed; no smoke, no bang, no sign came out saying I didn't follow instructions, nothing.

Even when I started working on other's computers, people would tell me about the 10 second rule.

So now, I've looked around trying to find out the origin of this rule among computers that I've kept in the back of my head until now and I haven't found anything. Maybe I'll try again tomorrow when I'm not running on 4 hours of sleep.