Separation Enamel is a specially chemically formulated enamel that, when painted on top of a few layers of fused enamel, will change the viscosity of the area to spread and show the layers below. The project in the book is written by Tom Ellis who usually uses this supplement with patterns to very nice effect. Others use it with non-pattern/abstract designs.
Here are some pieces made by using the book's instructions. Note the one who took a photo of the piece before firing, a great way to remember what you actually did!