Hiccups are muscle spasms in the diaphragm and throat.
The large muscle at the bottom of the chest cavity sometimes contracts in a spasm, as do most muscles at one time or another.
When this happens, air gets sucked in through the mouth.
The air never actually reaches the lungs because the muscles of the throat contract at the same time.
Any number of remedies have been suggested to stop hiccups, from swallowing dry sugar to standing on your head, but basically the same principle works here as with other muscle spasms.
If you relax, calm down, take your mind off the spasms, and get on with your activities, your diaphragm and throat will settle down and the hiccups will stop.