In this video Doug Marcaida explains the fundamental differences between attacking with a blade and dueling with an opponent who also has a blade.
The primary difference, he explains, is your ready position. In a knife attack the weapon must be concealed until the gap between you and your intended target is sufficiently closed to deliver an efficient, fight ending action. In a duel, where both parties are active participants in an inevitable fight or struggle the ready position must be adapted to extend your safety range.
Adopting a modified fencing position with your attack hand extended allows anything that intrudes that space to be cut. The distance between your opponent’s blade and your vital organs is extended and also protected by the extension of your arms.