User blog comment:Traffy731/luffy's red hawk attack/@comment-74.58.202.143-20130611055004

I also thouth like you at first but here's my theory: Luffy's second gear consist of accelerating the blood flow in his body. When doing that, it generates a lot of heat, hence the steam created when he enters second gear. I think his fire attack is some kind of enchenced second gear, in which he accelerates the blood in his body even further and thus, generating even more heat and creating flames.