One single gridded light on camera left.

Grids are honeycomb light modifiers that restrict and narrow the light. This is why the subject's arm goes into darkness.

Also in play here is the Inverse Square Law. This Law essentially states that light is brighter from it's source and becomes less potent the further it travels. That is why, when adjusting for even exposure, the amount of the subject that is lit directly ties to the distance of light to subject. 

For this image above, I put the subject close to the light. Adjusting for a really bright subject, means the remaining of the image will drop off into darkness: Inverse Square Law ftw!