Answer:
When surgical procedures are being done on the abdominal or pelvic area, a small incision is made near the navel, and an instrument called a Laparoscope is inserted. The surgeon uses a gas, usually carbon dioxide, and injects it into the abdomen. It is carefully monitored.
Some people have been known to do this as a fetish, by other means. However, it is very dangerous as a "do it yourself." It can cause intestinal shock, toxic shock, rupture, or even death. These would all be very painful, and not very quick.