Cool chap, Jimbe being the helmsman of the SH's has been speculated ever since all the lists with the SH jobs popped up every time a potential nakama popped up.
It really was the one thing that was missing to the ship navigation. Franky made it strong, Nami can predict the weather and Jimbei rides the currents. So badass he pulls the entire sail with his monster fists.
Don't exactly understand what Luffy did with the mirror or how it will help him out later on. but I'm sure it is his ticket out of there somehow.
Edit:
I've reread it and now I understand. Luffy covered his mouth to prevent Nami hearing him cry out in pain after Dogtooth kicked him. He did that so that they wouldn't worry. As soon as the mirror broke he was happy.
What a hero