User blog comment:B214/Volume 23 discussion blog/@comment-77.188.255.221-20170511231412/@comment-29963364-20170511235518

In the spoiler for vol. 23, Ajuka says this...

"Azazel has already found the answer for that. He says something along the lines of it hasn't appeared in front of you yet. And a fraction of that power may show up during the tournament."

The first sentance is referring to Azazel meeting Issei's future children during the events of EX. This is when Azazel learns of the beings from the other world, and that they will invade some time in the future. The ExE power that Azazel & Ajuka keep mentioning has something to do with these other world beings, and they might be the key to Issei aquireing this power.

Also, in the later chapters of vol. 21, Ajuka and Shiva reveal that Rizevim actually made contact with the other world, and even gave them instructions on how to by-pass Great Red and invade this world. He basically picked a fight with them and told them where to find him.

This is why Azazel thinks that "a fraction of that power may show up during the tournament."