This is the scene from chapter 25, where Watney describes how he had produced and stored hydrogen, which he produced by electrolysing water.
I need clarification of the bold part.
LOG ENTRY: SOL 529
I’m turning water into rocket fuel.
It’s easier than you’d think.
Separating hydrogen and oxygen only requires a couple of electrodes and some current. The problem is collecting the hydrogen. I don’t have any equipment for pulling hydrogen out of the air. The atmospheric regulator doesn’t even know how. The last time I had to get hydrogen out of the air (back when I turned the Hab into a bomb) I burned it to turn it into water. Obviously that would be counterproductive.
But NASA thought everything through and gave me a process. First, I disconnected the rover and trailer from each other. Then, while wearing my EVA suit, I depressurized the trailer and back-filled it with pure oxygen at one-fourth of an atmosphere. Then I opened a plastic box full of water and put a couple of electrodes in. That’s why I needed the atmosphere. Without it, the water would just boil immediately and I’d be hanging around in a steamy atmosphere.
The electrolysis separated the hydrogen and oxygen from each other. Now the trailer was full of even more oxygen and also hydrogen. Pretty dangerous, actually.
Then I fired up the atmospheric regulator. I know I just said it doesn’t recognize hydrogen, but it does know how to yank oxygen out of the air. I broke all the safeties and set it to pull 100 percent of the oxygen out. After it was done, all that was left in the trailer was hydrogen. That’s why I started out with an atmosphere of pure oxygen, so the regulator could separate it later.
Then I cycled the rover’s airlock with the inner door open. The airlock thought it was evacuating itself, but it was actually evacuating the whole trailer. The air was stored in the airlock’s holding tank. And there you have it, a tank of pure hydrogen.
According to my understanding, Watney is inside the trailer in an atmosphere of pure hydrogen. How, then, did he cycle the rover's airlock?
Which is the inner door he is talking about?