User blog comment:Winstoninabox/DS2 Hollow Invasions/@comment-6003396-20140206102330/@comment-6003396-20140206113455

Thanks for that link. I think I've sorted it out, with some help. There is apparently a distinction between "using" and "burning" an Effigy. Using one can be done anywhere and returns you to human form, restoring your HP, but you can still get invaded. Burning on the other hand can only be done at bonfires and does all the rest but instead prevents invasions and limits coop to some extent, which sounds like it will be on a timer, but I'm not sure.