User blog comment:12th Supernova/Chapter 784: Spring Theory/@comment-5239749-20150423224108/@comment-24.235.56.170-20150425233125

Just speculation, but I predict that the SBS which explains Gear fourth will tell us that Luffy is shifting the air in his muscles from one side of his arm to the other, which would physically curve his arm to change its path. Luffy is super innovative when it comes to his fighting style, so I feel like that would just be par for the course.