Manual switchoff of eMac display?

Nope, not that I've ever heard of. There might be a software hack for sleeping the screen on command, though I've never seen one. You could also physically hack the hardware, but that's way more trouble than it's probably worth.

If you do find a "sleep screen now" hack, that'd be cool, though--I frequently turn my CRT off under the same long download circumstances at home, but I won't be able to if I ever buy an ADC monitor.

Not so much as turning off the screen, but locking it (it runs a screen saver, which you can set to blank screen). In the Utilities folder there is an app call Key Chain Manager, or just Keychain. Open it and look through the menus for one called "Show Security/Keychain Settings on Tool Bar". You will now have a little lock on the menu bar, next to the clock. When you click on it, an menu drops, allowing you to lock the screen, and when you arrive back at your computer, you just type in your password, and you're back at work.

I did this all the time when I ran SETI @ Home. It was great, because I was able to run seti, at night with out the computer thinking it should go to sleep, or my waking up my computer while sleeping and quitting open programs.

TEG

All the "Lock screen" thing does is activate the screensaver. It's the same as setting a hot-corner, or just waiting for the screensaver to auto-activate. You aren't actually "locking" anything.