Today's video can be viewed (here) and gives an excellent treatment of this subject.
• the model is wearing dark clothing
• use a dark background
• as often the case the first recommended shot is with NO flash to check that ambient light has been excluded. Some photographers recommend that you should ensure that the background is underrexposed by at least 2 stops, just in case you want to boost the image in post. To be honest I've never done that!
• the need for lots of shadows in a low key image
• using a grid to restrict the area illuminated
Gavin covers two lighting scenarios:
• lighting from the front
• lighting from the side with a reflector opposite to separate the subject from the background