User blog comment:B214/Question & Answers 5/@comment-82.199.207.24-20160423084304/@comment-82.199.207.24-20160424083019

https://www.baka-tsuki.org/project/images/4/44/HSDxD_vol_01_007.jpg

And here too, isn't Issei concentrating demonic-power, to enhance the power of his punch?