User blog comment:B214/Question and Answers 6/@comment-118.172.205.177-20160711144501/@comment-26345952-20160712044857

Like I said the JD could at first use the negative emotion of the wielder to activate and then let the user activate it whenever they wish. However they will be consumed with negative emotions and use their life span as a source.

Vali is wxperienced because he has years of experience with his Sacred Gear but most importanly he uses his demonic power over his life so he can take control of the JD even if for a short period of time. The only two people that made Vali enter JD by rage were Tobio and Cao Cao. I see the case of Fenrir as both rage and will to use it.