The spasmotic contraction of the diaphragm muscles is the real reason for hiccups. The volume of the rib cage will be usually increased by the diaphragm to draw air into the lungs.
Most of us are unaware about how this happens as this goes away quickly. But, it should be considered as chronic and need medical intervention if it lasts for more than three days. There are many types of medical treatments published by many magazines.
Also, number of home remedies are also there for stopping hiccups.
A spoon full of sugar might stop hiccups. Basically, it depends on what you do with the spoon full of sugar and "it is not the sugar that stops hiccups". If a spoon full of sugar is deposited into mouth, we will probably become a little uncomfortable as to how to try to swallow it. The nerves in throat and oesophagus will be stimulated and will affect nerves of the diaphragm. So it should be kept in mind that most of these kind of remedies will be involving activities to put something into the mouth that can activate nerve reflex that affects the diaphragm.
When we put a spoon full of sugar as a remedy into our mouth, then the only thing that happens to stop the hiccups will just be a coincidence in timing as most hiccups stop quickly anyway.