My question is: It has been said that a teaspoon of either sugar or malt vinegar is supposed to clear the hiccups. From experience I have found that it does seem to work, but what I wanna know is if here is any scientific reason as to why this method might work?

Here's another recent thread on hiccup stoppage that may help:

 

http://www.scienceforums.net/forums/showthread.php?t=8605

I think you misunderstand, a tsp of sugar works for me, so i'm not looking for a way to make hiccups stop. I was just wondering if there was a medical/scientific reason as to why this method works?

Since hiccups are a spasming of the diaphram due to pressure differentials, I think it must be coincidental that breath-holding or sugar-swallowing seems to stop them. Perhaps it soothes you enough to relax the diaphram. Since they go away in time anyway, perhaps the time involved in finding the sugar, teaspoon or malt vinegar is sufficient.

I dont know but i was told once that instead of just holding your breath you should suck in as much air as posible and continue to try to suck in air once your "full" as to not let your diaphram spasm