Through the Viewfinder Photography

On Etsy I found a photographer who does something called "Through the Viewfinder Photography." She explains it as follows:

"Through the Viewfinder photography is defined as taking a picture of any subject through the viewfinder of any camera with another camera."

I think this is an interesting approach to taking a picture, and adds an alluring, vintage-y look to the image. Might be a new thing to try!