User blog comment:B214/Volume 23 discussion blog/@comment-30924344-20170319035400/@comment-10330822-20170319041430

Yeah, we still aren't entirely sure what kind of training Issei went through between Volume 21 and Volume 22. Chances are he is really close to reaching a new level, or perfecting DxD G.

In fact, based on the conversation between Ddraig and Issei, it almost seems like Issei can use the DxD G freely except for a time limit. Ddraig made no mention of the side effects that DxD G normally has. In fact, based on the conversation, Ddraig made it sound like Issei can use his pseudo DxD form whenever he wants with no real time limit, like CCP, and can use DxD G with a time limit.