User blog comment:B214/Volume 21 info & spoilers/@comment-71.205.30.36-20160407022108/@comment-24.113.129.70-20160408011924

I think I have a good idea as to how Issei's DxD Mode works, and how it gets nerfed in vol. 21. This might gonna be long, so please bare with me.

Sun Wukong stated in vol. 17 that Issei can't properly regulate how much power he uses when in battle. He almost always uses much more power than necessary and ends up burning through his stamina. But that hasn't been too much of an issue lately, as he's trained his body to handle that kind of power on a regular basis.

Now with his new DxD Mode, it's got a whole new variable to add to this "equation." So far, Issei's Boosted Gear worked by enhancing the abilities he already possesses, making his speed, strengh, defense, and demonic powers more powerful. How much he can enhance himself is limited by how much his body can handle using, and that amount can be increased with training and experience.

Now that Ophis has given Issei her attribute of "infinity," the DxD Mode gives Issei an infinite "power supply" that he can use to enhace his abilities. But since you can't enhance any further than "infinity," what the Boosted Gear enhances instead is how much of that "infinite power" Issei can tap into and use. And now Issei's problem of not being able to regulate his power is exponentially worse. Now that he no longer has a cap on how much he can enhance himself, he runs the risk of putting more and more stress on his body than ever before. This is likely what caused him to experience so much pain and go into a coma after using DxD Mode for only a few minutes in his fight with Rizevim Lucifer.

As for how Ophis nerfed this new form, I think she adjusted it so that it works similar to how it does in his BB and CCP forms, making it so that how much of that "infinite power" he can tap into and use is limited by how much his body can handle. With this adjustment, just like in his BB and CCP forms, he can increase how much his body can handle through training and experience.

If you read through this whole thing, I sincerely thank you. And I hope this answeres any questions you may have on the subject.