A very helpful book on all this is Master Lam Kam Chuen's Chi Kung: Way of Power. The book is well written, with amy beautiful pictures, Chinese calligraphy, and helpful illustrations. There is a complete long term system for developing Zhan Zhuang. Master Lam translates the calligraphy of Grand Master Wang Xiang Zhai to demonstrate poetically the essense of Zhan Zhuang.
Inwardly alert, open, calm.
Outwardly upright, extended, filled with spirit.
This is the foundation of stillness.
Add the hard and the soft. the powerful and the relaxed,
Motion and stillness, contraction and extension:
In the instant these converge, there is power.