a yellow square, indicating this line as the constituting ruler of the hexagram Six at the beginning means:

At the tail in retreat. This is dangerous.
One must not wish to undertake anything.

Since the hexagram is the picture of something that is retreating, the lowest line represents the tail and the top line the head. In a retreat it is advantageous to be at the front. Here one is at the back, in immediate contact with the pursuing enemy. This is dangerous, and under such circumstances it is not advisable to undertake anything. Keeping still is the easiest way of escaping from the threatening danger.