The head must be suspended.
Suspended head is an important principle of the Chen Style Taijiquan Practical Method. The suspended head allows the vertical stretch of the body to take place. This creates two lines in the body: 1) head left foot vertical line. 2) head right foot vertical line.
The other lines in the body from similar actions are: 3) left hand-right hand stretched horizontal line. 4) left foot-right hand stretched 45 degree line. 5) right foot-left hand stretched 45 degree line.
These five major lines in the body maintain the peng (掤）energy in the body. They also contribute to the concept of "five points struggle out", 五点外挣。
Source: https://practicalmethod.com/?p=37166