The fourth requirement to look into is that of the 3-passes.
The 3-passes is a mental visualization tool to guide the body to move about. There are 4 ways to use the 3-passes.
For practicing the 8-step Health Form we can simplify things by focusing on just moving forward and backwards. The two method of 3-passes that we will use are :-
a) Erect 3-passes (竖立三关)

The Erect 3-passes is used in the techniques of Wild Horse Parts Mane, Play the Pipa, Grasp Sparrow’s Tail, etc.
b) Retreat 3-passes (后撤三关)

Retreat 3-passes is used in the techniques of Play the Pipa, Repulse Monkey, Push, etc.
A more detailed explanation with video is available in the post here.