Observe that there's no boundary here. There's no shape. This space of awareness is just wide open. Whatever seems like a boundary, whatever seems to impose structure is itself just an appearance in this condition of awareness.
Space without boundary
In this meditation, Sam Harris aims at giving the listener/meditator experiential access to the unboundedness of the "space of awareness". This is the space in which all mental events (sensations, thoughts, emotions etc) arise and pass away when they are recognized as such by the meditator, and when the meditator then does not identify with them.
Sam Harris uses two strategies to make this experience available:
pointing out the contrast between the changing mental events and the unchanging awareness space in which they arise and pass
pointing out that the sense of boundaries it itself an appearance.
Consciousness itself is unmoved
The first pointing out instruction contrasts the ever-changing events with the changelessness of space, or, as Sam Harris calls it, consciousness/space/awareness itself is "unmoved".
Just let everything arise and change in its own place. And notice that consciousness itself is unmoved.
In Tibetan meditation literature, mental events have the character of liveliness (movement) and direction. In contrast, awareness itself has no movement and direction. It is boundaryless, changeless and timeless. It is always there (or here).
This is illustrated by various analogies, for example:
the analogy of the sun and its rays. The sun represents what is unchanged. The rays represent movement and direction of thoughts.
the analogy/ metaphor of the ocean and its waves. The ocean is what stays forever unchanged, while the waves come and go.
The changelessness of consciousness implies that it does not come and go in time.
However, Sam Harris does not point this out explicitly: in his pointing out instruction, the timelessness of consciousness is implicit. This approach is different from the "Pointing Out the Great Way" approach, in which timelessness is explicitly mentioned and one of the characteristics of awareness:
Boundaryless, changeless, timeless.
The limits imposed by the sense of having a head
The second pointing out instruction to give the listener experiential access to unboundedness is our felt sense of having a head from which we look.
And see if you can relax your sense that what you see is associated with your head. You don't see anything related to your head. There is just space.
Btw, Sam Harris's meditation uses the same term as "Pointing Out the Great Way" for the way, ordinary mindfulness acts on the perception of space:
In POGW language, the instruction goes something like this:
"If your ordinary awareness imposes any boundaries or edges...." etc,
An alternate strategy to remove the felt sense of boundaries: pour awareness into awareness
There is a third way in which the felt boundaries and structures of awareness can be "dissolved". This strategy is not used by Sam Harris but by "Pointing Out the Great Way".
This is the "mixing approach". In this approach, the meditator is instructed, to move with his awareness into those boundaries, or, to mix awareness with them, to pour space into space, to pour awareness into awareness.
This approach uses the idea, that the sensed boundaries are themselves only awareness. This is not dissimilar to Sam Harris, who also will point out that the boundaries are appearances in awareness or modifications.