A headstand in a handstand or a handstand in a headstand?
The pose is grounded in a triangle made by the top of the head and the two hands. As the hands can take pressure off the head and the head can take work out of the arms, the pose may be easier to hold at length than either of those standard inversions.
By raising the spine up between the shoulders (the shrugging motion that brings them closer to the ears) the pose approaches a handstand: you might even lift off the support entirely for a time. Conversely, by transferring some weight out of the hands the pose moves more toward the axial load bearing that characterises the headstands.
Usually I start by modulating slowly back and forth, pushing down more and then less into the hands; and then find and hold an equilibrium between head and hands for a period before coming down.
To work as described the support obviously needs to be the right height. To a degree it is possible to adjust by moving the hands out a little further out to the sides or in, or by bending the arms, slightly, as you come onto the support — but the arms must at all times remain capable of taking up all of the weight. It is possible to balance in the pose but it is inadvisible to attempt away from a wall: the support would prevent a safe roll down.
I come up with parallel arms, which is allows me to clear the top of the block, and then lower my head and adjust the arm position.
The arrangement of two blocks shown just happens to suit my proportions and the workings of my arms and shoulders. If it were any higher it would be troublesome getting above it and I would loose the ability to control the disposition of weight.
Other configurations of full (or half) blocks may suit others better. Wider bases are possible: I’ve done it on reams of paper stacked. Sadly telephone books are no longer arriving.
How to work out the right height of the support?
If using something light like our foam blocks, they can easily enough be mounted between the head and the wall in a box pose.
This position may be of interest in itself. We lose the shoulder mobility and the slidable spine that is so appealing in a normal box, but by leaning lightly into the support we can get in this three-pronged box a taste of headstand’s axial activity. This could be done just as well with a single block, and could also be transferred into any standing pose where the torso is, or can be, horizontal: warrior three, triangle, half-moon, pārshvottānāsana.
Half handstand, i.e. with feet kept down on a chair, is a fuller way to work out the right configuration for the support. This is already a significant inversion: it too can be done as an end in itself. Easier to get into, it is also more strenuous than the lifted position, one of whose main characteristics should be readily accessible lightness.
Heading in the other direction, headstand can be further loaded: the tripod headstand takes more weight onto the head than what has become the basic form, and then there is the muktahasta (‘released arm’) variants (plates 199-201 in Iyengar’s Light on Yoga). And beyond that, no hands and arms up by the waist: good for a job with the circus, which is just as well since there may not be another good reason to do it.
Image on bottom right: detail from c. 1820 'Company School' painting in the British Museum.