This video is woefully outdated. The Panel Loops feature is a much better way to accomplish this task. If you are interested, this method still works, it’s just a little old-school. Quick overview of the Create Difference Mesh function used to add extruded thickness to a single walled surface geometry without leaving the ZBrush environment. Extrudes like this on low resolution geometry are easier using the ZModeler brush now, but for dense meshes, this method still works better.