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The skull meets the spine at the atlanto-occipital (AO) joint. If you put your fingers in the indentation behind your earlobes so they're horizontal and imagine a line connecting them, the AO joint is on this line. Nod gently and you should be able to feel the articulation.
The structure of the joint itself is interesting. The occiput, which is the bottom of the skull, has two kidney-shaped bony structures called occipital condyles on it. These sit on cup-shaped structures (facets) on the atlas, which is the top vertebra (C1). https://t.co/312JboaySW
These are remarkably small structures considering that your head weighs between 4.3 - 5.3 kg. Doesn't feel like it, right? That's because your head maintains a dynamic balance as it tips forward and back. But actually the centre of gravity of the head is forward of this joint.
This is why when you're very tired yet still sitting, your head tips forward as you start to fall asleep. There are a different sets of muscles between your spine and skull, both in front and behind, that keep the system in balance: intrinsic and extrinsic.
Intrinsic muscles are smaller, deeper, connect higher up on the spine and are harder to consciously control. They make fine, subtle corrections. Extrinsic muscles are bigger, nearer the surface and much easier to consciously control. They also connect further down the spine.
Through Alexander Technique we see that most people over-use their extrinsic muscles and under-use their intrinsic muscles. When they try to use their intrinsic muscles, they end up using their extrinsic muscles more. (I won't list all the muscles here right now)
But you can access these smaller muscles indirectly. One way, weirdly, is all this awareness stuff I talk about. It's this process of 'getting out of the way' that allows them to get involved. Let's see if a little game will work. Bring your head to more or less level...
Move your eyes down and gently allow gravity to articulate your head forward a little (don't just drop your head). At some point you may notice a subtle lengthening process along the length of the back of your neck. This lengthening gently wants restore balance to your head.
If you notice this, gently allow it to bring your eyes back to level. This should be am easy experience. If you try too hard to do this you will use the extrinsic muscles. Once you've returned to level, notice the gentle dynamic balance as your head rests on those condyles.
Living with this joint unlocked, rather than held by the bigger, less finely controlled extrinsic muscles, is a much more pleasant way to move and live. As an experiment, go for a walk and see if — without trying too hard — you can notice the subtle movements in that joint.