apart from flushing out drug metabolites, which is the main use of diuretics, they can also dramatically reduce body weight in a short space of time by reducing the amount of fluid you retain. now for your average punter, this will show little effect in performance but for those at the top of their game where the difference between gold and silver is measured in 1/100ths of a second, that small weight loss can help increase speed over short distance. over long distance it's a negative effect due to the dehydration effect of diuretics so has little use for long distance athletes.
many many years ago I used to sell a very powerful diuretic used for patients with high levels of oedema - often a side effect of CHD. we used to sell an injectable version which looked like water and was tasteless and odourless. for a laugh we'd occasionally break a vial open and drop it into someone's drink - sure enough, the drinker would be off for a pee very quickly, and again, and again, and again etc...