User blog comment:B214/Question & Answers 5/@comment-82.199.219.93-20160317194727/@comment-26345952-20160317200123

it's the wielders own body. That power output would put a heavy burden to wielder and when the body can't take any more boosts it will break and put a lot of pain on the wielders body. (watch episode 11 of season 1 or go to vol. 2 of the LN for a better explanation)