Yeast are a living organisms that break down sugars and releases gases(carbon dioxide) into their surroundings. Humans have been using yeast for quite some time to make bread rise or brew beer. Some feel that since yeast is a living thing that can grow and multiply, some people(or all people) may have an over abundance of yeast in their system. This could possibly lead to all sorts of health problems, which sound eerily similar to IBS.
Usually these diets involve avoiding yeast products entirely, as well as taking probiotics & vitamins to combat the yeast overgrowth. Again this will take some commitment as yeast is all around us.