This is Rembrandt lighting. The best clue is the triangle shaped light on her right cheek.
If Rachel was sitting straight up, you would have Rembrandt lighting with the light source above her and to the right or left. You know the light is in the right spot when you see the triangle of light on the cheek opposite the light source. Since she is leaning, to get Rembrandt lighting the light source is close to straight up and her face is angled slightly toward the light. The light was provided by a studio strobe pointed almost straight up and bounced back at Rachel with an umbrella.
Rembrandt lighting is a popular way to light portraits. To try it, put your light source above and to the right or above and to the left of your subject. Move the light around until you see a triangle of light on the cheek opposite the light source.
You can also use fill light on the side of the face opposite the main light, but reduce the power of your fill light source so the fill light doesn’t wash out the triangle of light.