You can't turn off the monitor of an iMac (just that). It has never been possible. Either sleep, or with the screen on. If you want too because you attach a serious external screen, the only thing to do is to just disconnect internal wires and hope it keeps working.
Turning off your iMac display, at will and instantly, is much easier than most people suppose. It is nothing more than a keyboard shortcut. This is somewhat amazing considering all the lengths people go to to accomplish this (setting screen sleep to 1 minute in power settings, using a completely black screensaver, or even developing software!)
1
CONTROL + SHIFT + EJECT. That's it! These 3 buttons, press simultaneously (you can press them down 1 at a time, but they must all be down by the end of the command) will cause your "display" (not computer) to "sleep", which turns it off.
2
Simply shake the mouse or press any button to turn the screen back on.
3
If you want to set your iMac to actually "Sleep" (not just the screen), just activate it in the "Energy Saver" pane in the "preferences" area.