The idea is to get an out-of-focus dark edge to separate light toned things from their surroundings. It takes two exposures and two light sources.
First, open the background light - the subject appears as a dark silhouette. Shift the lens-board slightly forward to enlarge and defocus the subject. Expose.
Second, close the background light and open the foreground light. Shift the lens-board back to normal focus. Expose again.
It looks horrid when it is overdone. Hope this will be useful.