In case you're as confused as anyone not named Rosa or Chuck might be, Rosa is using an old automotive repair trick. She wants to weld up the hole in her bike's fuel tank that stranded her out here in the first place, but fuel fumes plus open flame is a bad combination. That's where the third leg of the "fire triangle" comes into play and mechanics will use various methods of making sure the tank is an oxygen-poor environment, one of which is pumping it full of exhaust from a running car, or in this case, a running generator. It probably goes without saying, but: don't try this at home.