I've been pondering this thread, because the question is a delicate one, IMO. Went back and reviewed some notes, in fact, and had a "think" about it for a while...so, here's the result.
Objectless Samatha is about undistractedness. One maintains awareness of "now," and doesn't allow oneself to get carried away by trains of thought. In that sense, I agree with Andrew108. Even though we say it is "without object," it is still a meditation method "with focus." It is skillful means. It requires effort, and has an "outward focus" even though it is nonspecific, really. When we get carried away with conceptual thought, we let go of that, and return to a simple awareness of "now." Sounds, appearances, smells, sensations, and thoughts arise and disappear, and we "see" them but do not get distracted from this initial "seeing," or carried away into conceptuality about them. That is the method of Objectless Samatha, in a nutshell.
Resting in "the emptiness of mind," as you put it, or, as I would put it, resting in Mind's Essence, or we could say "resting in Ordinary Mind" which is a Mahamudra term, requires first that you KNOW this state of Ordinary Mind, that you've had a decisive experience of it, usually as a result of Pointing Out Instructions (though in rare cases, not...) and that you are experiencing that state, an aware, nonconceptual, empty-yet-cognizant, wide-open state.
The experience of this state is usually said to be quite brief, for beginners. The goal is to sustain the continuity of this state--but it's a tricky business. The practice is sometimes described as "Undistracted Nonmeditation." Training is involved, for sure...one doesn't just sit there and allow oneself to wander off into fantasy-land, or allow oneself to be drawn into conceptual thinking, trains of thought, etc. That's not what is meant by "Nonmeditation." There is also a danger of making this State an "object." If that happens, you're back to a conceptual practice, an "objectless Samatha" in a sense. As the Third Karmapa said in his "Aspiration of Mahamudra," there's nothing to see. "It is not existent-Even the Buddhas have not seen it."
There's a lot more to be said...but better to hear it from a teacher who holds lineage and blessings.
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