So Kull wanted to cut off Charlotte's arm? It was a fair plan, but a doomed one.

First of all, Kull's comparison of Charlotte's attack to a jab was logical, except in one manner; while it would take momentum to deal damage with a punch, it would not take momentum to inflict the pain desired by putting force on the wound. Therefore it was a simple matter for Charlotte to simply slide her thumb down into Kull's forehead and into his eye. In fact, she probably could have adjusted course mid attack, but it was too late for that now.

The sword. The sword was still a problem. But, again, there was one mistake in his logic. High school physics: when pushing a lever, the further away from the fulcrum you are pushing, the less force is necessary to move the lever that far. The lever being the gladius, and the fulcrum being Kull's hand, this would mean that his additional force from momentum could be rendered irrelevant if Charlotte could meet his blade at the middle with the handle end of her blade, and, considering that the wide blade of the cinquedea was meant for parrying anyway, she could feasibly stop the gladius in its tracks. So, that was what she did.