It's Natural Infant Hygiene! To summarize, you should eschew diapers completely and rely on reading the baby's elimination cues, thus rushing to hold them over the potty or a bowl. This process is called "peeing the baby." What's even better is that you make a sound (the article suggests "ssssss") to associate with the peeing so that the baby learns to pee when he hears the magic word. Hey, that sounds familiar. Isn't that the way I trained Scout to pee outside?
via Jennifer Weiner