Ittou Shura

"One minute was enough. It was fine to ignore what happened afterward, but for one minute he'd become strong enough to beat anyone. That was the answer Ikki Kurogane found, so that his greatest weakness can defeat another's greatest strength. Intentionally using all of his vigor and stamina after breaking through his limits, it was a Noble Art that brought out all of his meager power for little more than one minute and multiplies that power several dozen times."

- Tne narration describing Ittou Shura.

Ittou Shura is the Noble Art of Ikki Kurogane.

Description
Ittou Shura is the Noble Art, in which Ikki is able to amass all his bodies vigor and stamina while ignoring the survival instincts that prevent humans from using their full strength, then multiplies this several dozen times granting him tremendous power, which can be maintained for just one minute and as such, it is referred to as his strongest minute.

During his match against Touka Toudou in the finals of the Selection Battle Arc, in order to defeat Raikiri, Ikki had used an improved version of Ittou Shura, Ittou Rasetsu to defeat her.

In the Seven Stars Sword Art Festival Finals, as Ikki fought against Stella using Dragon Spirit, he had reached the peak of his potential, which would've taken decades for him to realize and reached his Awakening not wanting her to find another rival, further increasing his magic power to where he could still use Ittou Shura despite one minute already passing and had enough to perform Ittou Rasetsu soon after.

In the Vermillion Empire Arc, training with Edelweiss, Ikki attained better magic control, improving his Ittou Shura, despite not being shown, the former had stated that he could make it three times as strong as before. Ikki, on the other hand, theorized that he might be able to regulate Ittou Shura in combat, while the magic that would end up being dispersed can be applied to Intetsu. If he were to achieve this, then his Ittou Shura would evolve, making it potentially as powerful as Ittou Rasetsu.

Weaknesses
Ittou Shura expands and drains all of his stamina and strength for a minute, so Ikki will be unable to fight using it for an extended period of time. Its usage is limited to once per day due to the incredible strain that it puts on him and has visited the hospital a number of times after using it, while at times having enough strength to make it to his room. This is a weakness that a number of opponents have taken advantage of as shown by Ayase Ayatsuji who wanted to cripple his chances of winning.

Though the power granted by Ittou Shura is tremendous, it doesn't grant him a guaranteed victory. It is first shown in his initial fight against the Twin Wings and World's Strongest Swordsman, Edelweiss who proved too strong for him to match leaving him on the defensive. In his fight against Stella Vermillion in the finals, he only managed to match her using Dragon Spirit but was not able to outclass her.

Through his training with Edelweiss in order to improve his magic control, he was able to overcome some of the limits of Ittou Shura and improve it, but by how much was unknown.

Trivia

 * The secret technique among the Shishou is similar to that of Kirinkou, in which both entails ignoring their survival instincts to release their full power.
 * Ittou Shura is a reference to an alternate name for the Asura, a low-ranking god or demigod in Buddhism who are more powerful than humans, but chained down to the cycle of rebirth by great egotistical passions, more so envy.
 * Ikki himself could be related to the Shura in several ways as the lowest-ranked among all Blazers, but he wields strength far above a human chained by his own fate (until becoming a desperado), seeking greater power.