Double-clicking the icon empties the coffee pot (that's what the icon is) and temporarily disables the program. The icon is shown above - it's the leftmost one in the task tray, and this is all you see. It works by simulating a keypress once every 59 seconds, so your machine thinks you're still working at the keyboard, so won't lock the screen or activate the screensaver. If you have problems with your PC locking or going to sleep, caffeine will keep it awake. Caffeine Prevent your computer from going to sleep
