This is from the offical site;
Calcium carbonate and magnesium carbonate are commonly used as antacids in a variety of preparations. They alter the acidity of the stomach's juices, by binding to excess acid. This decreases the acidity of the contents of the stomach which may in turn help to relieve the symptoms of ulcers, heartburn or dyspepsia. Simethicone is an 'anti-foaming agent' which reduces the amount of trapped wind in the gut. It works by allowing the gas to be released as larger bubbles rather than multiple small bubbles, which are trapped among the stomach contents.
....and I can understand how this works quickly in the stomach but I can't work out how it works in the Large Intestine as quickly as it does. Doesn't it take ages for something swallowed to get to the other end? Just take a Tums or Rolaids and a Dulcolax. It's the exact same thing, and probably a hell of a lot cheaper.
And no, it doesn't take ages for something that you swallow to get to the other end. Try eating corn sometime and then timing yourself. No | 