User blog comment:B214/General discussion/@comment-108.224.166.119-20160125030409/@comment-24742995-20160125174739

Basically, everyone(well, not everyone actually) is tied to the Fate that binds this world. As Ouma puts it, a person's magical energy determines one's power to affect Fate. Ikki, with such low magical energy, has little power to affect fate.

Spoiler alert:

During the end of the final match between Ikki and Stella, Ikki used his third secret sword to redirect Stella's impact back to her, and clashed with her repeatedly in Ittou Shura. During that time, Fate(or maybe will of the world) decided that Ikki has reached his limit, and indirectly hampered his movements. As Ikki puts it, he seems chains on his legs and he started losing to Stella. But Ikki in the end, because his love for Stella and he wants to keep on being the person that Stella is chasing as a rival and not somebody else more talented, broke these chains out of sheer force of will, at the same time becoming someone that is outside the Fate of this world(similar to Edelweiss and a number of other people introduced in Volume 9). Because of that, his magical energy also become boosted, and this allows him to enter Ittou Rasetsu(even though he has already used Ittou Shura once), defeating Stella with it. Even Edelweiss is shocked that Ikki was able to awaken at such an age.

In short, Ikki "awakened" because the world told him that he is fated to be a wimp and Ikki basically "fuck you rules and talents" out of sheer force of will to the world because he is so badass.