Nitrous oxide started as anesthesia, but found a nice second career as a drug for anyone who could find a can of spray-on whipped cream. Why does it hit people so suddenly? Why does it stop so quickly? The answer may be found in biochemistry and physics that are unique to this particular gas.
Nitrous oxide was invented in the late 1700s, by a man named Joseph Priestley who first tested it on himself. Although it should have jumped to surgical use right away, considering how badly they needed pain killers in those days, it first made the rounds at carnivals, where adults and children paid vendors to take hits of it. They stumbled around, to the amusement of the crowd. It wasn't until one of them injured himself by stumbling and didn't feel any pain that a local physician began working to put nitrous oxide to medical use.
When it was used in medicine, doctors noticed an odd quirk to the way it hit people. It was faster than nearly any other kind of inhaled anesthetic. It was more effective than its chemical properties would seem to suggest. Why was this? They looked into it and found that nitrous oxide caused what they called the "second gas effect."
The gas you regularly inhale is made up of mostly nitrogen and oxygen. Oxygen trips across the barriers in the alveoli in the lungs, and enters the blood stream. Nitrogen generally doesn't (although it can be forced into the blood stream through fatty tissue, when the body is under pressure). Nitrous oxide, however, does a great job of sprinting across that alveolar wall. It does this so well, in fact, that the pressure inside the alveoli drops, and they contract quickly. The alveoli, in turn, squeeze down on the remaining gases inside the lungs so hard that the gases are concentrated, which further speeds the uptake of the nitrous oxide. Scientists found that the reverse was also true. The nitrous oxide left the body quicker than other inhaled anesthetics.
This combination of biochemistry and physics is one of the reasons why the effects of nitrous oxide on the system are so sudden, and why they decrease so rapidly. This worked for carnival barkers, and whippit enthusiasts, but it's effects are still being studied by anesthesiologists.