User blog comment:Rappy 4187/Technical Update: August 31, 2015/@comment-5558012-20150901003221/@comment-126761-20150901122059

It will be a simple checkbox at this point - but that doesn't rule out making tweaks to it in the future. The interpretation of it as a security checkpoint is correct - we want to make sure users explicitly opt-in to the feature with clear information on what it does.

When it goes live, personal JS will initially be switched off for everyone - but we will be endeavouring to contact everyone who already uses personal JS to let them know that they just have to read a few quick guidelines and flip a switch.