http://www.runnersworld.com/drinks-hydr ... orts-drink
So what do we know about how this actually works? As Jeukendrup's review points out, there's no metabolic reason that carbohydrate ingestion should help during short bouts of exercise (e.g. an hour or less). Your body doesn't run out of carbohydrate during such short bouts, and the exercise is over before any significant amount of the extra carbohydrate you swallow is actually absorbed and oxidized. To drive this point home, Jeukendrup and colleagues did a study in 2004 where they infused glucose (or a saline placebo) directly into the veins of cyclists performing a 40-K time trial (which takes about an hour). In this case, the cyclists actually could use the extra carbohydrate, but it had no effect on performance. The muscles simply don't need it in such a short event.
Instead, it's the brain that's influenced by mouth rinsing: fMRI studies have shown that certain regions of the brain light up when you have carbohydrate in the mouth, whether it's sweet or tasteless. Artificially sweetened (but carbohydrate-free) rinses don't produce the same effect, so it's not a taste thing, and it's not consciously mediated. In the review, Jeukendrup notes that these observations are consistent with the existence of a "central governor" that regulates motor output based on signals from the muscles -- and from other places, including the mouth.