This effect when producing photographs in the darkroom with film can be created by exposing a partially developed negative to white light before finishing the developing process.
For this you need a white light source (ideally a speedlight) exposed film (I will be using 4×5 large format) and developing kit.
After photographing the subject onto the film, you set up the development stage as normal (developer, wash, fix) but when wanting to produce this effect, you partially develop the film, expose with a speedlight for a fraction of a second, then continue developing as normal.
The photograph should solarize where some of the shaded areas turn lighter and lighter areas turn shaded, almost producing a negative image in parts of the picture. Along with this a dark line should appear along defined edges on the photograph which will show more when it is processed onto paper as a final photograph.
This effect can also be created digitally by using the curves tool in a photo editing software package and creating a sharp “u” or “v” curve and adjusting slightly from this point to mimic the above effect.
The below two images are a before and after in paintshop pro to show how the effect can be digitally made. ~Note how the darker areas have appeared lighter and vice versa with lighter areas appearing darker. (Apologies for the model but it is a self portrait!)