To practice Tai Chi, Ba Gua, or even Qigong without a real grasp and feeling of Qi, is to go through the motions with an empty shell. The Qi moving through the moves, and in between the moves; is what gives the internal its power. Without Qi, these are no more than a pretty dance; as a car is when an engine is running in neutral.
The hollow shaft of a metal arrow can deliver some martial force; however when that tube is filled with a propellant and payload, it becomes a jet or missile.
This book's focus is the development, feeling, testing and using Qi yourself.