User blog comment:B214/High School DxD BorN Episode 9 Review/@comment-23847729-20150601192532/@comment-67.172.91.107-20150602032221

Well Vali is also the only other current possesor, so as far as living characters it's a pretty small sampling size. I guess it is never answered definitively one way or the other, but the way Odin says "They [previous possessors] activated ‘Juggernaut Drive’ when they felt like it," makes me think that they would have to become familiar with the power and how to control it to some extent. Also the fact that the other characters who know about JD seem to be more surprised by the manner in which he reverted (poking Rias's breasts) than by the fact that he was able to revert at all, which, if he was the only possessor to ever have survived Juggernaut Drive asside from Vali, it probably would have been mentioned.