Shapes and Morphing - Blender Foundations: The Essential Guide to Learning Blender 2.6 - page 285

shape, click the + button. A new key is created, reflecting the visual state of the masked smile shape key,
but without the use of the vertex group.
This is an important point. Using the + button creates a new key, based on the visual state of the mesh.
You can set the Value sliders for several shape keys at once, and their effects accumulate on the object,
allowing you to test different combinations. When creating a series of keys for facial expressions, you should
always make sure that you create new keys with the + button only when all of the Value sliders for other
keys are set to 0.0. Fortunately, there is a shortcut that resets them: the “X” control located on the far
right, just above the name field.
The ability to create a new shape from the current appearance also lets you experiment with different
combinations of your existing shapes. As you change the Value slider for your different shapes, you might
run into a look that you particularly like. Saving it is as simple as adding a new key and naming it.
Figure 10.4 Too much smile, and a smile turned upside down.