Krakauer actually talks about that exact event twice in the novel. The first time is very early on in the book when he is getting his audience's attention. The second time is about halfway through the book when he is telling his readers a more detailed, chronological occurrence of the events on the mountain.
Krakauer is stuck at the top of the Hilary Step on his way down from the summit. The problem is that he can't go down it, because a bunch of climbers are coming up it. It's a huge bottleneck. Krakauer can't do anything except wait. Andy Harris arrives shortly after, and Krakauer asks him to turn off his oxygen. It's dangerous, but Krakauer wants to save what is left of his bottled oxygen for when he is later physically exerting himself.
Andy is experiencing hypoxia at those altitudes, so his thought processes are not as sharp as they should be. He makes a simple error with Krakauer's bottled oxygen. Instead of turning it off, Andy turned the gas on all the way. For about ten minutes, Krakauer felt great. That makes sense, he was breathing a lot of oxygen. Unfortunately though, the mistake used up all of his bottled oxygen, and Krakauer was forced to descend large parts of the mountain without supplemental oxygen.
Wanting to conserve whatever oxygen remained in my tank, I asked him to reach inside my backpack and turn off the valve on my regulator, which he did. For the next ten minutes I felt surprisingly good. My head cleared. I actually seemed less tired than I had with the gas turned On. Then, abruptly, I sensed that I was suffocating. My vision dimmed and my head began to spin. I was on the brink of losing consciousness. Instead of turning my oxygen off, Harris, in his hypoxically impaired state, had mistakenly cranked the valve open to full flow, draining the tank.