GST(iv) – GST(vii) on page 138 extends our understanding of the push hands game by teaching the idea of control and position in that first you control, then you get into position and after that do what you like.
In this case this movement teaches the principle of piercing the legs of the opponent, something which I have not really seen or heard in other Taiji styles. For beginners, this principle is difficult to use because their lower body will not listen to their command.
It is only much later, after a lot of practice that the advantage of piercing the legs become obvious. An example is on page 143. If you have been having problem getting your training partner off balance using techniques such as Wild Horse Parts Mane, try doing it with piercing legs principle.
The application from page 263 – 267 shows how GST(iv) – GST(vii) is used with piercing legs. Page 266 pictures (ix) – (x) and page 267 pictures (xi) – (xiv) shows the process of using piercing legs.