(A/N: Not my work but I get a lot of questions about this and he explains it a lot better than me.)
All rights to Dr.Dexter from Fetlife-
SUBSPACE
Within the context of BDSM, "subspace" is a an altered psychological state that is often entered into by the person bottoming in a scene. It is not easy to characterize, because each person's reactions to BDSM play can be quite different -- and even a single person's reactions to play can vary from scene to scene.
Most people associate BDSM with the physical aspects of the scene: The floggers, whips, rope, and the like. However, there are many mental aspects to a scene as well, and those need to be considered during every scene.
Subspace, in one sense, is much like an hypnotic trance. A trance is any period of narrowly-focused attention. If you've been "sucked into" a book or movie, such that the world disappeared for a time, you've been in a trance. During a BDSM scene, as you progressively focus more and more on the physical sensations of play, the entire world may disappear, leaving only you, the Top, and whatever is being done to you.
At the same time, BDSM quite often involves impacts to the body. During a scene, the intense experiences of both pain and pleasure trigger a response of the sympathetic nervous system, which causes a release of epinephrine from the adrenal glands, as well as a dump of endorphins and enkephalins.
These natural chemicals are a part of the body's primal "fight or flight" response. They produce the same effects as a morphine-like drug. This dump of morphine-like chemicals into one's bloodstream into increases the pain tolerance of the submissive as the scene becomes more intense -- and also induces a euphoric, ecstatic floating feeling.
Subjectively, subspace is like getting drunk or getting high on drugs. You forget the pain, your problems, all your cares seem to drift away and be obliterated while you're in this state. Some submissives, upon reaching a height of subspace, may lose all sensations of pain, or become incoherent, making safewords useless.
The euphoria of subspace (or some parts of it) can last anywhere from hours to days after play. Some people can carry a positive "glow" from play that lasts them for weeks.
However, it is also a state of mind that impairs rational thought and decision making skills. It's a state that needs to be monitored carefully for the mental and physical safety and well being of all the parties involved. While it is important to take this into consideration during play, it is especially important to remember this as a scene is winding down and is stopped.
If a submissive goes far enough into subspace, they could injure themselves without knowing it, or continue to ask for play that could injure them without their knowledge. If the Top whom they're playing with doesn't understand the dangers of subspace, it can be even more dangerous for the sub.
The experience of subspace is a major reason that subs play within the BDSM world. Aside from exploring desires they've kept hidden from themselves, and experiencing a form of sexually-related play that operates on levels of explicit communication that they've probably never experienced before -- learning to fly in subspace is a powerful and ecstatic experience. It is literally an incredibly powerful "natural high".
