The Quicksword is performed by channeling all of one's yoki into one arm, essentially letting that arm Awaken. In essence, the user's arm "goes berserk with full Yoma power while the rest of the body strives with all of its might to control it." The end result is a barrage of sword swings with such great velocity that it is impossible to see with the naked eye. Irene does not even appear to draw her sword when using this technique, especially evidenced in her battles with Teresa and Ophelia.
Using the technique requires a tremendous amount of concentration to contain the yoki within the arm alone, as the natural instinct is to release yoki throughout the entire body. Irene's superb spirit and unflappable personality makes her well-suited for the technique. Her mastery of the technique is such that she does not even show the slightest hint of exertion while executing the move, even for prolonged periods of time.
The Quicksword was first seen against two Yoma when Irene rallied with Priscilla, Sophia, and Noel in a Yoma hive in order to commence the mission to kill Teresa.Not only did she manage to kill the Yoma without appearing to draw her sword, she was subsequently free of bloodstains despite the fact that the Yoma were cut open directly above her.
During the fight between Ophelia and Clare, Irene intervened and saved Clare as she was about to be killed. The Quicksword technique was so fast and controlled that it completely deflected Ophelia's Rippling Sword, a boon that even Clare's pre-emptive yoki sensing ability could not provide beforehand. Later, Irene taught Clare the technique so that she could remain formidable despite losing her dominant arm. At the end of the training, however, Irene explained that the technique was not compatible with Clare. The Quicksword requires perfect calm at all times, especially during battle, and though Clare outwardly portrayed such stoicism, her heart was "a whirlpool of passion." In addition, due to Clare's status as a quarter-Yoma warrior, her strength and speed were far less than the minimal requirement for an effective Quicksword assault, being at most a tenth of Irene's. As such, Irene gave Clare her remaining arm so that the latter could perform the technique at half of Irene's speed and power and with more control.
Though Clare originally lacked Irene's ability to control the Quicksword, she would later learn to control the technique by combining it with her pre-emptive yoki sensing ability, effectively making the Quicksword respond only to the Yoki of her target.
Quicksword and WindcutterEdit
After the first encounter with Awakened Beings in Pieta, Flora was quick to notice the similarity between her own technique, the Windcutter, and Clare's Quicksword. Both techniques work with a split-second's high acceleration of the sword, delivering an incredible number of consecutive attacks.
After the fall of Pieta, the Seven Ghosts had to hide their auras, meaning Clare could no longer use the Quicksword freely as it required yoki. Given the similarity of the Quicksword and the Windcutter, as well as in honor of Flora, she took it upon herself to learn the Windcutter.