User blog comment:B214/Volume 23 discussion blog/@comment-31137559-20170531193743

I kind of figured out why Vali called Issei's JD incomplete. JD allows the possessor of BG and DD to forcibly release the power of the two Heavenly Dragons thus putting them at that level or at least close to it. In the process, the negative emotions of the past possessors get released, driving JD. And the possessor is swallowed hp by those negative emotions, losing themselves in the process and going berserk and rampaging until their entire life force is used up. All there is in JD is the negative emotions because the possessor has already succumbed to those negative emotions.

But with Issei, those negative emotions weren't the main thing driving him. It was the shock and pain of apparently losing Asia again. He activated JD because of that. Now those negative emotions were still a factor, but over all that was the shock and pain of losing Asia. That was way more than those negative emotions. So Issei was still in there. Normally, Issei should have lost himself in the abyss of the negative emotions. But the shock and pain of losing Asia was far stronger than those negative emotions, proving Issei was still in there. That was the reason why Issei was able to get out of Juggernaut Drive.