Sculpting - Blender Foundations: The Essential Guide to Learning Blender 2.6 - page 225

Figure 8.1 The basic head, with sculpting tools shown.
● Flatten: “Squashes” geometry toward the center of the model.
● Clay: Builds the mesh up as though with layers of clay.
Shaping Existing Models
Back when we discussed character modeling (Chapter 5), one of the techniques for generating a head was
to start with a default head structure and push it around to match reference images. While you can do
this in Edit mode using the O-key Proportional Edit Falloff (PEF) technique, a more intuitive way to
work is with the sculpting Grab brush.