Rembrandt Lighting takes its name from the style of painting by Rembrandt, no surprises there. Apparently Rembrandt worked in a studio with a skylight which would cast a light on one side of the models face, the style was characterized by the triangular pool of light cast under the eye on the dark side of the portrait, this was caused by the shadow of the nose on one side and the side of the face falling away from the light source.
The key light should be level with or slightly above the line of the nose and approximately at 45 degrees to the model. It takes a bit of trial and error to get it exactly right when doing this in a studio so experiment with the height and position of the light until the effect is achieved.
In Set A Light I found I needed a flag or light blocker on the opposite side of the model to stop the bounced light filling the dark side of the face, and true to form the same proved to be true in real life, demonstrating the accuracy of this modeling tool. In real life I hve a set of stand mounted reflectors that are rectangular and I made black covers for them which work really well for this.
Here is the basic setup which mirrors almost exactly what I had to do in the studio:
And again into the studio to put it all into practice, Its worth noting that Rembrandt Lighting is more suited to a male model as it is quite a harsh lighting scheme and not designed to flatter the way some of the other schemes do, thus it is a rugged lighting that suits the persona of a man: