User blog comment:Bennlimos/HxH Questions & Thoughts Discussion 2/@comment-16938192-20140305000335/@comment-14902757-20140305072249

2) I get it now. Questions 1&2 are actually related. The pin inside of Killua's head was imbued with Illumi's nen. But since it's inside Killua's head, even with gyo, it remained invisible to others because Killua's skin on his forehead (and his skull) served as the "wall" which is the concept I'm trying to discuss in Question #1.