NOTE: you probably want Control instead of Command —

this image is for MY machine which has those buttons switched

Ho ho, so I like to lock my screen when I leave my computer, and I couldn't find a hotkey for it on my mac, so I made moving my mouse to the lower-left corner bring up the password protected screen saver, but I always felt like I needed to wait a second, because if I moved my mouse right away after the screen saver started, it would go away without asking for a password..

..but I learned of mashing control, shift, fn, and the power button — which on my machine is command instead of control since I switch those buttons — which instantly puts the computer in a password protected state :)