How to get screen-lock on demand, e.g. when walking away from a machine still executing something:
<<<OS X has native screen-locking built-in; though you wouldn’t know unless you dug around. To enable it, open your Keychain Access utility in the Applications / Utility folder. Under the “View” menu at the top, select “Show Status in Menu Bar.” A black padlock will appear in your taskbar in the upper right-hand corner. Close Keychain Access. Now when you click on the padlock, you have a “Lock Screen” option in the drop-down. Selecting it will cause your screen to fade to black and engage your screen saver. Click any key to open a login / password dialog box, and you’re back in.>>>