The authors describe a study that explored the mechanism by which a carbohydrate mouth rinse, without ingestion, may improve performance. In that study, magnetic resonance imaging studies were performed and it was noted that supraspinal pathways of the brain were activated by either a glucose or maltodextrin rinse (6.4% in either case). These pathways were related to motivation and reward during exercise. One of the most surprising findings came from one cycling study in which there was either ingestion and mouth rinsing with the same carbohydrate dose. In that study, only the mouth-rinsing, not the ingestion, improved performance. The performance improvement was attributed to above mechanism coupled with lowered requirements for blood supply and energy cost for digestion of absorbed carbohydrate.
Another important point made by the authors is that, in the two studies that did not show improvements, subjects consumed a standardized diet followed by 2-4 hours of fasting. In the other studies, fasting periods were typically longer. They speculated that the nutritional status of the athlete (e.g., liver glycogen stores) might be an influential factor.   However, it was noted that some studies still showed positive effects with as little as 3-4 hours of fasting, a time period that is generally too short to fully deplete liver glycogen. Sweetness did not appear to be an important factor, as 4 of the 6 studies used maltodextrin, 1 used sweetened maltodextrin, and only 1 fed glucose.