User blog comment:B214/Question & Answers 5/@comment-26345952-20160224225639/@comment-195.195.244.67-20160225100825

Issei's JD was incomplete because he didn't have wings. Vali's did so that's how you know his was incomplete. The incomplete state only released half the power of Ddraig. The complete state like Vali's unlocks the full power of the Heavenly Dragon so that means he can kill a God when in that state. To use the full power forcefully when activating JD you have to be in control. Issei being too weak to handle it doesn't make a bit of sense as that means Vali wouldn't be able to use JD if it was apparently too much for him. The only reason why Vali can use JD without dying is because he uses his massive magic reserve in his life force's stead.