So, last night I finally got a chance to try some of the exercises that required me to use the viewfinder. These consisted of balancing the viewfinder on my left hand while tracing the edges of my hand with my right hand. Oh, and keeping one eye closed the entire time. I had an easier time closing my left eye and focusing with my right eye, for some reason. You also need to rest your hand on something comfortable.
I used a non-permanent black marker to trace my hand on the glass. The picture isn't bad, considering that it took me about five minutes to do. But the goal of the exercise is to help you draw a foreshortened viewpoint, and I didn't feel that this particular picture capture that at all--my middle and ring finger were actually bent, the glass pane balanced on my fingertips.
This one came out a little better.
The basic idea is that the glass sheet with the cross on it becomes a physical manifestation of the virtual picture plane that trained artists naturally project onto three-dimensional objects when trying to draw them in two dimensions. Using the viewfinder is all part of training the mind and eye to see with an artist's eye.
It was an interesting exercise. I have to say that it did give me a bit of a headache, but it allows for very quick drawing of the basic outline of a shape. I need to practice this a bit more before moving on to the next stage.