I’ve thought a lot over many years about how exactly the forward and up direction of the head operates. As a quick way in with new students I usually point out that the powerful neck muscles which connect the head to the torso mainly attach to the back of the head, in the area between the backs of the ears, and that even includes the muscles (sterno-cleido-mastoids) which begin on the front of the chest. The reason being of course the uneven distribution of the weight of the head. Therefore I ask people to think of (or “ask for”) an upward release through the back and sides of the neck to let the back of the head lighten up away from the back and shoulders. Accompanied with my hands that seems to work pretty well.
On a more sophisticated level, I think the forward element of the forward and up of the head is only comprehensible when it’s seen in relation to the back and up of the whole spine. It puzzled me for ages why Patrick Macdonald said in his book that the head is to be directed horizontally forward at 90 degrees to the spine whereas some other first generation teachers described it more as the head nodding forward when the muscles attaching to the back of it are released. But it makes sense experientially to me now that so much of what we deal with in life is in front of us and below our eye line that the whole spine is constantly dragging forward and down, particularly through its curves. As the whole spine comes more back and up, the head is naturally riding in a more forward relationship to the spine without any need for nodding forward. When things work well the whole column of the spine is “stiffened” in a good way – the “elastic bracing but not rigid bracing” Walter Carrington sometimes talked about, borrowing terms from the horse riding world – and because of this firm integration the forward tendency of the weight of the head pulls on the whole spine. This could result in the spine leaning forward, or it could result in the upper spine curling forward, but either result would entail a loss of height. The combination of asking for that forwardness to operate on the spine but refusing to lose height has instead the effect of bringing the spinal curves into better balance with each other, and in particular can reduce excessive cervical or lumbar lordosis. The forward head weight brings the back/spine back and the backness of the back/spine enhances the forward relativity of the head.
You can experience some of this in sitting and leaning forward and in monkey provided the knees continually release forward to counterbalance the backward pull on the spine, but it can be most powerfully experienced in walking with these directions in mind. Keep an upward tendency and rib movement through the back for breathing and the forward is less a nodding than an interaction of the forwardness of the weight of the head tending to stimulate the spine to come back and vice versa. It takes some time to build up while walking but it’s worth it!