How do I turn off display on MacBook Pro when connected to external display?

I've recently purchased an external display made by HP and I've got a problem. When I've got it connected to my MacBook Pro, I'd like to turn off the display on the MacBook Pro completely, as it's a waste of power. I've tried turning the brightness all the way down, but that doesn't actually turn off the display. Also, I've tried many Terminal commands found around the web and none of them seem to work. Does anybody know a solution?

Any answers are appreciated greatly. 🙂