The Demonic Sword had a flexible structure. It could compress and divide itself according to Noah's needs, but it needed to return solid during specific actions.
Noah couldn't use the Demonic Sword to cultivate when it wasn't in its original form, and he needed it in specific shapes to perform his martial art in the past.
Yet, the Cut had sharpened his individuality and made him find his original expression of his power. A sword could slash in any form, even if it were nothing more than a short knife.
Noah didn't want the Demonic Sword to suffer due to his physical strength, but there wasn't a way around that issue. He could either hold back while performing different slashes or change the living weapon's shape to increase its density.
In the form of a short knife, the Demonic Sword would be able to endure more pressure without affecting its sharpness. Noah only had to change his slashes slightly to tune it with the new length of the weapon.