Those who were divided and did not put their all into something were usually weaker against a full-on offensive, especially in cases where someone was only able to defend themselves using one of their arms while the other was preoccupied.
Their arm was shielding them, but it was partly blocking their vision in the process.
That let Ash react quickly to deal a devastating blow that would break past his defence and deal the most damage.
She could have turned her attention toward the knife Stephan was reaching toward, but the moment that she halted her attack and tried to stop him, it was possible that Stephan would notice the change and would take that opportunity to counterattack once she was distracted.
With his movements being disrupted and him stopping for long enough from pulling out the knife or other potential weapon, Ash was able to shift her body a little as she reached toward the arm.
