Awareness itself is wide open and totally undefined. Simply rest as that and watch your thoughts liberate themselves.
Sam Harris uses a strange thought: how can a thought "liberate itself"?
Sam Harris references a Mahamudra/Dzogchen concept: the self-liberation of thoughts, which is connected to the stage of meditation of non-meditation.
Another preferred expression for the same concept in Sam Harris´ meditations is "unwinding". A thoughts unwinds, unravels. In older Buddhist texts this is compared to a snake uncoiling.
The concept of "self-liberation" of thoughts is key to understanding how meditation becomes effective. Also called "automatic emptiness" in Tibetan meditation, it captures how the mere awareness of a thought as it arises destroys its ability to "grab" the mind, and thereby to create any lasting imprint as a memory trace (called "karma").
Instead, thoughts appear as "manifestations of awakened wisdom", regardless of their content. Thoughts are the appearance part of the "inseparable pair" of ground and appearance (the space of awareness, and the mental events that occur in this space).
Various metaphors describe how one can imagine self-liberation:
a snowflake melting in a warm ocean,
a drop of water dissolving on a hot rock,
mist melting in the sun etc.
This process is called "automatic" because eventually nothing must be done to achieve this, except letting it happen. "Self-liberation" is one of the most frequently used terms in Daniel P Brown´s "Pointing out the Great Way". Here is one quote:
"Once the correct view has been established, nothing needs to be done to bring about liberation from emotional suffering. Emotional states become self-liberated (rang grol) as they arise. As emotions arise, awareness-itself shows them to be nondual, relative manifestations of awakened wisdom" (D. P. Brown & Thurman, 2006, p. 514)
Brown, D. P., & Thurman, R. (2006). Pointing Out the Great Way: The Stages of Meditation in the Mahamudra Tradition (Annotated ed.). Wisdom Publications.