This technique is inspired by what was possible way back using film, by exposing the same position of the roll for several times. Doing this, enabled re-exposing the dark parts of a film to a new light. (assuming they were dark and unexposed on the first exposure). Of course doing this well, required lots of film and lots of patience.
With Photoshop, this technique is not expensive anymore and has infinite possibilities. The idea is very similar: overlay a second image on the dark parts of a first one.
This is how it’s done: