I won’t even attempt to explain what through the viewfinder photography is because I don’t really understand it personally myself. Instead all I can say is that they’re shots taken through the viewfinder of another (much older) camera like the duraflex. Someone correct me if I’m wrong.
What I CAN say however is how I love the products of this photography style. The pictures come out slightly out of focus and the colours are either wonderfully super saturated or washed out. Here are a few examples I found from the flickr Through the Viewfinder group: