User blog comment:B214/Question and Answers/@comment-50.11.177.49-20130708052828/@comment-14738352-20130807051758

Just like DBard said, they grant the wielder the power of the Heavenly Dragons, but in a berserk state; Doesn't that also mean that they can't maximize the use of their power?

So you can say that when the Issei and Vali deviated from the path of the Juggernaut Drive to their own, they can control the power of the dragons more.

BTW, Vali obtained the Juggernaut Over-Drive by accepting the urge to destroy or the "curse" of the previous Hakuryuukou's.