Ok, so now we move on to my thoughts on Gear Third.....
Alright, a brief explanation. Luffy bites into his thumb, and blows as hard as he can into the bone of his thumb, inflating it. Since the bones are harder, the effect is similar to an inflated car tire, and is very dense and therefore powerful. To move Gear Third throughout his body, he transfers it to his chest and then to a different part of his body.
Ok, so a few things need to be considered here. Human bones, unlike birds, are not hollow. They do have small pores, similar to a honeycomb, but they are pretty damn small. This seems a bit odd to me that Luffy can inflate something so small like this. The only reason that the Gomu Gomu no Fuusen works is because his lungs are naturally made to store air. In addition, if his bones are hard enough that they can punch through walls and level giants and pacifista, biting through your bone and filling it with enough air is pretty crazy. Not only would Luffy pretty much be cutting his thumb open every time he does this, but it would take a tremendous amount of lung strength to provide enough air pressure for this to expand. Remember that Luffy's body is similar to a rubber band, so force must be applied for it to stretch. The amount of force to cause his bones (the hardest things in his body) to expand would be huge, especially with air.
Second thing. Luffy transfers the air between limbs using his chest, which is fine in theory. But if the air really is sitting in his bones, it wouldn't work, and here is why: bones in the body are not all connected. The bones in his arm aren't the same bone as the ones in his chest. So unless the air can jump between bones, this seems pretty much impossible to me.
Thirdly, what is the deal with the downside of Gear Third? He shrinks? I hardly have any theories for that. Maybe it is because he forces all of the oxygen in his body to one area, and when he release Gear Third, he has no oxygen for a while? But is that even possible?
Finally, after the timeskip, he doesn't seem to have to bite into his thumb or anything. How the hell can he use it then? As if the previous theory wasn't hard enough t understand, he doesn't seem to give air to his bones at all! Hmmmm......
Anyways, I could really use some insight, so if I screwed something up, please feel free to correct me. Thank you for the insight.