User blog comment:B214/High School DxD BorN Episode 9 Review/@comment-23847729-20150601192532/@comment-67.172.91.107-20150602025056

I don't believe Juggernaut Drive was ever said to be totally irreversible, just that it is a tremendously taxing state for all but the most talented and experienced possessors (e.g. Vali, Belzard and Elsha). Odin tells Issei in Volume 7 that past possessors would activate JD frivolously whenever they wanted. I believe, since Issei is repeatedly named the weakest/least talented possessor in history, that not only could he just manage an incomplete transformation, but even that was so physically demanding for him that it almost led to his death. Keep in mind that he had only recently achieved balance breaker and his ability and stamina at this point still required countdowns and time limits.