Hiccups are involuntary contractions of the diaphragm. These contractions cause a sudden inrush of air, slamming shut the leaf-shaped flap of cartilage in your throat known as the ‘epiglottis’ (this is what causes the ‘hic’ sound).
The documented cures for hiccups generally fall into three categories – raising the level of CO2 in your bloodstream (holding your breath), stimulating the vagus nerve (sticking your finger in your ear – or anus!), and distraction (a sudden fright). The technique of drinking from the wrong side of the glass may be a combination of the last two: it requires you to concentrate on the task, and cold drinks stimulate the vagus nerve through the stomach wall. If you drink a large glass without stopping, you may even interrupt your breathing for long enough to affect your blood CO2
However, none of the folk cures for hiccups have good scientific research to back them up, and scientists don’t understand the mechanisms that might be at work. Most studies are anecdotal reports of cures for patients with persistent bouts of ‘intractable hiccups’ that have gone on for months or even years, and it’s possible that these have a different underlying cause. For more ordinary episodes of hiccups, there seems to be a large placebo effect at work – so as long as you believe in the cure, it will probably work.