Head upright means that the head must be held perfectly erect, (i.e. suspending the head) so that the vital spirit can reach the crown of the head. One must not use force; if one uses force, the muscles of the neck will be strained with the result that qi and blood will not be able to circulate freely. Rather, one must have a light and alert feeling. If the crown of the head is not suspended lightly and alertly, the vital spirit cannot be raised.

Sensei’s commentary in italics…

This is the key to form ward-off. Hollow the chest so the qi can sink to the dan tien. With the energy sinking to dan tien, ward-off will make you centered and rooted. One must avoid forcing out the chest; if the chest is forcibly expanded, then the qi will accumulate in the chest with the result that the upper part of the body will be heavy, the breathing will be shallow and the lower part light; the heels will be too light and consequently you will be uprooted. If you can manifest ward-off correctly, you can store qi and be ready to emit power through the spine.

In the book T’aiji Classic, it is said “the root is at the feet and power is generated from the legs, controlled by the waist and expressed by the fingers. From the feet to the legs to the waist must be integrated, and one unified qi.” The waist is the controlling axis of the entire body. If one can relax the waist, then the feet can develop rootedness and strength and one’s stance can be secure. The interchange of substantial and insubstantial all derives from the rotation of the waist.