The technique of taking a picture with a Fuji Instax instant camera, waiting for it to develop, and then taking a picture with the Instax photo in the frame as close to where it should be as possible. This creates almost a see-through effect. – Instax Windows

If you want to try it yourself, find some more inspiration in the group pool.

Photos from lintmachine, Slightlynorth, poopoorama, and TrevinC.