Subspace is a mental state where a person becomes mentally relaxed, dreamy, floaty, disconnected... which some describe as a natural high.
In books and blog posts subspace is often described as being achieved by masochists who enjoy pain, where, it is commonly said, pain creates endorphins, and the flood of endorphins take the brain into subspace.
(Endorphins don't actually cross the blood-brain barrier, so it may be that this is better understood to be what it feels like to be taken into subspace through pain, rather than an actual physical description of what's happening in the brain).
Other people find they can enter subspace purely through psychological means, without anything physically intense going on.
A tone of voice, or a tug on the hair, or the simple focus and attention of practicing BDSM can take some people into subspace.
Subspace is a trance state, where the conscious mind relaxes, and the subconscious becomes predominant.
Another word for this is “hypnosis” — where we’re talking about an actual real-world hypnotic trance as opposed to how hypnosis is portrayed in fiction.
There’s nothing unusual about a trace state, as everyone experiences trace states everyday. Someone can be driving, for example, and “zone out” and drive on automatic without need for conscious attention unless something unexpected happens.
It’s normal for the the conscious and the subconscious to be constantly trading off control during the day. You can be walking, for example, while consciously paying attention to every detail of how you’re walking and consciously deciding where to place each step... or you can be paying attention to something else, walking automatically, letting your subconscious handle moving your legs without you having to pay attention.
A deeper trance state, one that we might call a “hypnotic” trance or “subspace”, is one where the conscious mind more fully checks out.
So what makes a trace state “subspace”? After all, lots of people can experience trace states without then feeling particularly submissive.
The difference is in the person’s desires.
For someone with a desire to be submissive, to relax and surrender... a trance state is an opportunity to do that.