User blog comment:Monkey.D.Me/PUNK HAZARD - FAQ/@comment-4685278-20120216090107/@comment-4812569-20120216094908

Oh ok. I think they have explained why he produces steam when he uses Gear Second. Search up 'Gomu Gomu no mi/Gear Second techniques' and read the first few sentences under History. Should explain it. If you're talking about the 'Red Hawk' move Luffy used when he was fighting Hody then no one knows. Lots of theories out there though.