Diagram of a tub-shower diverter

When you pull on the handle a flap or little valve blocks the flow of water to the spout, making it run to the shower head Long story short, since I've spent the entire day at different hardware stores, and websites looking for the answer, and figured it out... before taking the advice of so many and replacing the entire spout.. The little black rubber washer is the value. When water pressure pushes against the hole, it actually pushes this black valve out just enough to stop the water from flowing, and the water flows to the shower head. If water is pouring out, try cleaning this black valve with toothpaste, as someone on another website recommended, and reinsert the valve. But don't push it all the way in. Water need to get behind this valve in order to fuction properly. Hope this answers the question...