Jean (The Organization's former No. 9) is the creator of the Drill Sword technique. After the seven-year timeskip, Helen (the Organization's former No. 22) is shown to have mastered the technique in honor of her former comrade.
The technique entails having the user wind up her sword arm and then release it, creating a drill effect. The end result is an immensely powerful attack. Galatea described it as the "attack with most velocity and striking power."  It has been shown to be able to pierce Awakened Beings with incredibly tough armor, such as an insect-like Awakened Being in Pieta, Dauf, and even hard rock.
Jean has shown a variation of the technique that involves her sword spinning like a propeller. This was achieved by orienting her blade perpendicular to the long axis of her arm before releasing her wound up arm.
Jean was shown as being able to twist her arm up to 21 times, using up almost 50% of her Yoki in a single powerful strike. Helen, however, managed to reproduce this technique without expending yoki and was able to twist her arm up to 55 times. This is likely an effect of the natural flexibility of her arms. Her flexibility also allows her to do the attack without the prolonged preparation time. According to Claymore Databook 2, the Drill Sword is the most powerful attack but also, the slowest one, due its preparation time.