Ciaoscuro is all about the play between dark and light.
Take this student at Vistek, the other day. Lit from where the camera is, you get this:
Fine, I suppose… competently lit, just barely.. but is that creative? Not really.
Now, lit from the side, with a simple flash with a grid on it, no other modifier, we get this, instead:
I think you will agree that’s a lot better, and for several reasons. One is that there is less stuff. Only what’s important is lit: the rest is simply not lit at all. Second is that the face is now shaped (modeled) by the light. Third is that what is important is lit; what isn’t is simply not lit. Light direction as well as distribution and quantity are now totally under your control.
(Note that the grid is essential: without it, the flash light would spill onto the walls and ceiling and floor and from there to the rest of the room: no black room)
What I used? A 5Diii with a 600EX flash on the camera set to be master (but not to fire itself); and a 430EXii slave flash on our left. that’s all. “Studio setting” (1/125, 200 ISO, f/8) ensures that the ambient light is black.