Board Thread:Questions and Answers/@comment-26047416-20150127010029/@comment-14902757-20150404130347

My answer to the Dragon Dive thing remains the same. (Unfortunately I can't find the link to that past Q&A). Since Killua knew the name of the technique, that kinda implies that he was told about it in the family while safely avoiding the details of nen to be divulged. He didn't have to see dragon dive first-hand but tales of it may be floating around in the family. Remember Killua was telling Gon of a story about his gramps infiltrating Republic of East Gorteau (ep98)? That means he had time with his family, he had time for stories, etc...

Regarding Illumi's needles, when Killua discovered the one in his forehead and realized that he was under his brother's control, it's not impossible to think that  the same can be done to weaker individuals with less regard to their lives.. From there, he's free to deduce what else can be done with the needles.