Reverse painting as it relates to pen making has already been nicely described so I won't go there, but the background on the phrase "reverse painting" is worth a short description.
There is an old art form, most commonly found on vintage Christmas ornaments, that involves painting in reverse on the inside of glass globes. The artists would actually insert a tiny brush inside the hole in the globe and paint a scene in reverse on the inside. Some were quite elaborate and are very expensive. Visualizing and then painting a scene in reverse is quite a feat. Imagine reversing the order in which the paint is applied as well as flopping the image. You would have to paint the details before the background, the shadow before the object is shadows, the eyes, nose and mouth before the face, etc.