on page 12, when he annouces that he will do something again because blueno took the "jet bazooka" attack,
luffy sticks his thunb into his mouth like he's trying to trigger something for the next gear

im thinking for each gear, there is a trigger to start it
like for his "gear second," he pushes down on his legs

itd be interesting to see how many gears he has, and how he will deal with himself not being use to the "gears" yet