Cross lighting: the model is lighted from the one direction and the background from the opposite direction. Optimally one should get a dark edge of the model against light background and a light edge against dark background. This lighting theme can be found from many old paintings by Rembrandt (ex.1) and Caravaggio (ex.2). In early photography at least Nadar used the technique (ex.3). One of the best applications on photos I've seen is a photo by a Finnish portrait photographer Jussi Aalto (ex.4) who used it on his portrait book cover. Cross lighting is typically done with two diffusors but it can even be done with one. With two the set up is pretty obvious - just opposite sides, one for the model, the other for the background. Barndoors, honeycomb can be used with the bg light to define the gradient. If only one diffusor is available a curtain or something is needed to act as barndoors to create the gradient at the background. It can be pretty iterative to optimize angles and distances. I know that Jussi Aalto can do it easily with one light. I am not skilled so I tend to do it with two. You will find three examples of mine my attached here.