This was one of the great advises I received from Aiko San, it was in one of the national championships, she pulled me to the side, and told me: “try to imagine your head as a ball resting delicately on your shoulders, if your head lean to either direction it will fall this way, if it is to fall, it should fall between your legs.”
Than she make me do yori ashi with Gyaku Zuki, first slow and than at full speed, and at kime, I had to check if my head was to fall to either side or between my legs, to be correct my neck had to be totally relaxed.
We did this for over an hour, I forgot that I was there for competition, and looking back this little advise was worth more than winning 5 national championships.
When the head just rests and floats on your shoulders, in addition to improved posture, lengthening the spine and reducing compression, there are lots of benefits: the tension in the neck and shoulders is reduced, the breath and the energy can be condensed and lowered to the Tan Den (lower abdomen), so our power is coming from where it is supposed to, and the power can be delivered effectively to where intend to, without escape of energy.
As we deliver energy we store energy in the center, which is not possible if there is access tension and top power.
Aiko San gave us those small, wise advises that had huge influence.
I use this advise regularly in my training and teaching.