Wet on wet is watercolor at its best! Many wonderful things happen during this stage. This is the time when soft, seamless transition in color and value happens.
When you paint a shape wet on dry, you’ve created an area for watercolor to flow. As long as you paint within that area, the paint you drop in will not spread outside. This is beneficial for creating color and value variety within a clean shape. I do that quite often with car windshield, skin, and other shape that has soft transition in it. You can also create a soft edge by pre-wet the paper with clean water. Always remember to wet the area bigger than the shape you are going to paint. That way, the paint you drop in will not reach the border of the wet shape thus creates a sharp edge.
If the paint is still wet, you are free to change to shape. Make the area bigger and make the shape more sophisticated as needed. Just keep in mind that the bigger the area, the more challenging it gets to monitor the overall wetness. Part of the area can be still quite wet while another part can be drying up. Drop in some wet paint to keep the area wet if needed. As long as it's still wet, watercolor will remain alive and able to flow freely.
Be mindful and observant with the wetness of the shape. If the shape is drying pass a certain time, it's best to leave it along until it is completely dry. If you put wet paint during that stage, it's going to explode and creates “cauliflower edge”. And you don't want that if you want to keep it clean. And that means to leave the painting as is even when it didn't turn out exactly the way it wants. It is better to let it paints itself instead of trying to fix it and making it worse!
One technique I use is to soften the edge with clean water. What I do essentially is to extend the boundary of the wet area with clean water, so that the color inside will spread, but it won't reaches as far, thus creates a soft edge. This is very useful if you want to paint something with both hard edge and soft edge. Such as distance buildings, some soft shadows.
More than 1/2 of the painting is done during the wet on wet stage. Although at the end people tend to focus on the fine detail, what makes the detail works so great is the beautiful washes underneath. Remember water is the limit! within that area, magic happens! Enjoy this stage.