That's known as the "earth blade" position in some circles. with you arm extended and your thumb side of your hand facing up, the blade would be pointed down with the cutting edge facing foward.We had a guest instructor at one of our classes. He tossed me a training knife and said to attack him. I pulled the blade down against my forearm with the handle in my fist. The instructor laughed and said now if you ever see anyone holding a knife like that run. This man was an expert and he was serious. Then he told me to handle the knife like most people do. Even bringing a gun to a knife fight does not guarantee you will not get cut, unless you are willing to unload your gun quickly from a distance.
From this position, you can punch, cut, slash and stab with basic motions. You're also protecting the blade from being taken away.
A lot of disarms rely on getting under the blade and applying leverage. With the knife in this position, you've taken those types of disarms out of the equation.