Whilst Vol 3 will cover the push hands game, the actually fundamentals are laid out in Vol 2. Page 137 is where we begin our journey for studying the basics of push hands game.
GST(i) – GST (iii) covers the cultivation of awareness (i.e. proprioception) where we know how far our mind can be extended for use in push hands game. This is the beginner’s level. When your study is sufficiently far enough then you must extend this study into that of how to use the turning to neutralize opponent’s attack. If you go further on you can also extend its use into that of mapping the areas of attack and defense.
Yes, this movement doesn’t look like a big deal but therein lies the rub. Most of the simple movements actually conceal a level of complexity that is largely a study of the mental processes. This is why Taijiquan as an art is not a monkey see, monkey do type of activity. Many of the principles are taught by personal transmission. As Yang Chengfu said if such principles are not mentioned even if one studies ten lifetimes one would still not get it.