If writing a novel is a year's exile to a foreign country, writing a short story is a weekend spent somewhere exotic. They're much more like vacations, more exciting and different, and you're off. "Look at me, I'm writing something that I will finish by tea time!"Though he doesn't write that fast all the time (later in the interview he reveals that a "pornographic" story had him stuck for four months), I think I can understand the rush of writing a short.
The spark of an idea appears; you try to hold on to it for as long as you can; if possible you play around with it; turn it this way and that; see what can be moulded out of the idea. Then you grab pen and paper, or keyboard and word processor, off you go! - exploring a little world that's approximately 3000 to 5000 words long; before you know it, time bends in extremely complex ways just so it can deceive you by running faster than it ought to. In 4 hours that feels like 4 minutes, you've finished a short story. It may not be a good one, but you had fun writing it anyway, which is the most important part.