I dont think the exercise is as mysterious as you make it sound.
Basically to do the exercise you start to move but you do not this creates a "force" moving forward.
You do the same for all directions.
standing on stake builds the muscles to be able to withstand the posture for a long time, helps correct body alignment errors, and help to relax parts of the body we were not aware was tense.
the moving but not moving force in the directions builds the minute muscles to work in those directions.
Then you also get a sense of changing directions.
The same thing is practiced with San Ti Shi in Xinyiquan.
Now its just a matter of putting it into practice to understand it.
appearantly some teachers put thier hand on your shoulder and try to hold you there, and see if you can move. If you can move you developed the proper force for Hun Yuan Li. so its now just a matter of practice and testing.