@Fire Fist:
Wave your hand in front of your face rapidly and watch the image. Though it's a blur, you see an afterimage of where your hand was, right?
That's what Asura was. Zoro moved so rapidly that it appeared that there was 3 heads and six arms. Kaku couldn't dodge it and got sliced open.
Although that is a decent interpretation of Asura, I'd have to disagree with you. There is a philosophy that if you wish to beat someone, you have to have the desire to kill them to truly defeat them. Before Asura, Zoro only wanted to fight him because of the key (which, I might add, was the actual key to free Robin), not to seriously kill him. But after Kaku insulted her, Zoro filled with enough rage to emit an aura of Asura, which frightened Kaku enough to lower his guard and become beaten. It had nothing to do with moving one's body so fast that he had an afterimage. One, that would really hurt his neck and arms to keep that up for so long, even being as trained as him. Zoro's intensified rage was so pronounced, that it created the feeling of seeing the Goddess of Death and Destruction embodied in him, meaning he was serious enough to kill Kaku, unlike before. With this aura, Kaku was shaken up, so his Amenadachi was possibly not the same strength when he used against Zoro and was able to be dissapated by the attack, and was ultimately defeated.
As for Gear 3rd, your bones are relatively hollow. They're filled with yellow and red marrow. It's in the spongy red and yellow marrow that Red and White Blood cells are produced. Luffy's bones are unique because his organ system is Rubber, which would make his blood rubber as well.
Regular Fuusen is normal respiration. Because of Luffy's Rubber body, his lung capacity is probably ten times anyone else's in the series. That, and it owes part of his lung capacity to shonen logic. Bone Fuusen, however, forces a large amount of air into the bones, which is distributed among the bones in his body through the circulatory system, before being dispelled through his lungs.
Also a good explanation, but not for me to agree with. Firstly, do you know how hard it is to bite into your bone? It's impossible. Gear 3rd is created by Luffy biting into his thumb so he can gain access to his bloodstreams and blow air into that. His blood vessels expand with air and enlarge his body parts as he moves the air along his body. I do not know how he expels the air, because he can obviously talk while doing his, but it is possible that if he can be wounded enough so the air is eventually force out of his vessels, or maybe his mouth was bleeding and it came out of there at some point.