Two Kinds of Subspace
Subspace is a widespread experience among BDSM subs, and from my own interactions with partners and discussions with friends, it manifests in two distinct forms. I have only seen one of these discussed in published materials and online forums.
What I call physiological subspace, which is the variant most commonly described, is induced through sustained pain play, especially from impact toys, e.g. a bare hand, flogger, paddle, cane, etc. It is apparently caused by endorphins, adrenaline, or other body chemistry. The actual sensation varies among individuals. The way it was described to me by a partner experiencing it is a strong whole-body buzzing sensation coupled with an inability to focus her eyes. Others I've spoken with have described it as a warm, floaty, spacey, serene feeling which is less bothered by pain. Some people attach spiritual significance to the experience. For some it is the point of kinky play.
Physiological subspace is not inevitable during S&M play. I know of one top who would monitor his partner and pause when she felt subspace coming on, until the feeling passed, so that she would remain wholly present, and fully sensitive to pain, during their sessions.
What I call psychological subspace, which I had never seen discussed in writing before I witnessed it first hand, occurs far more commonly than the physiological kind. It is apparently induced by feeling drawn to a dom. I have no idea what causes this phenomenon, but it is widespread among subs I have met. It is not the same as sexual arousal, though may be accompanied by that. The sensation is very much an altered state, where the person becomes inarticulate or non-verbal, suggestible, and feels floaty and warm, possibly with mild tingling throughout the body. It is an inviting feeling, and hard to resist.
Problematically, a sub can drop into psychological subspace around a dom she hardly knows! I've experienced this very situation, during a first conversation with a girl, on two occasions — once on the phone, and once sitting together at a cafe. In both cases my companion was caught off guard by the sudden, intense feelings, and somewhat overwhelmed. Fortunately this state can be managed or suppressed, and it's very important for subs to learn to do so, as subspace leaves a sub vulnerable to a dom who's motives she isn't certain of. Simply breathing and bringing your attention back to what's going on around you will help lift you out of subspace, but you'll need to sustain that effort once you come back to the surface.
Although I have no experience of hypnosis, in either therapeutic or erotic situations, I suspect that psychological subspace is sometimes mistaken for a hypnotic trance. While it's conceivable that they're related states of mind, psychological subspace is not caused by a dom, merely catalyzed by him. I have to wonder whether doms who claim to practice hypnosis are merely triggering subspace responses in their subjects!