I'm pretty sure that wasn't actually an attack, but Hisoka using the intensity of his aura to intimidate Gon and Killua so much that it felt like they were being physically blocked, but it was all in their mind. Kind of like how Wing used his aura to freak Killua out so much that he had to jump back all the way across the room. After seeing a face like
this, I don't blame him.
Then again, I'm really not an expert on the intricacies of Nen, so it's entirely likely that I'm completely wrong about this. That's why I love the way Togashi made Nen so different from other typical shonen energy forces like Ki and chakra. It's so complex that you really have to pay attention to grasp all its ins and outs.
---------- Post added June 18, 2013 at 02:16 AM ---------- Previous post was June 15, 2013 at 11:50 PM ----------
Surely someone here has a better answer to the OP's question than me? I find it hard to believe that mine was completely correct.